    """Abstract supertype of [[categories|Category]] whose
       elements may be iterated. Iterable categories are often
       called _streams_. A stream is a source of [[Iterator]]s,
       which produce the elements of the stream. A given element
       may occur more than once in a stream, that is, it may be
       produced more than once by a given iterator of the stream.

       A stream may have null elements. That is, an iterator for
       the stream may produce the value [[null]] one or more
       times. For every non-null `element` of a given stream
       `it`, the expression `element in it` must evaluate to
       `true`. Thus, a stream is a `Category` of its non-null
       elements.

       A _finite_ stream is a stream whose iterators are
       _exhaustible_, that is, they eventually stop producing
       elements. A stream need not be finite, but its elements
       must be countable. That is, for any given element of the
       stream, every [[Iterator]] of the stream must eventually
       return the element, even if the iterator itself is not
       exhaustible. It is possible for a given element to occur
       a (countably) infinite number of times in a nonfinite
       stream. It may not, in general, be possible to even
       determine if an insteance of `Iterable` is finite.

       For a nonfinite stream, certain operations of this
       interface either never terminate or result in an
       [[AssertionError]].

       A stream may be _mutable_, in which case two distinct
       iterators for the stream might not produce exactly the
       same elements. Furthermore, even an immutable stream
       might not have a well-defined order, and so the order in
       which elements are produced by the stream's iterator may
       not be _stable_. That is, the order may be different for
       two distinct iterators of the stream.

       However, a stream has a well-defined set of elements, and
       so any two iterators for an immutable finite stream
       should eventually return the same elements. Furthermore,
       any two iterators for an immutable finite stream should
       eventually return exactly the same total number of
       elements, which must be the [[size]] of the stream. For
       an immutable nonfinite stream, every element returned by
       a given iterator must eventually be returned by any other
       iterator of the stream.

       A stream may be known to be _nonempty_:

       - The type `Iterable<Element,Null>`, usually abbreviated
         `{Element*}`, represents a possibly-empty stream.
       - The type `Iterable<Element,Nothing>`, usually
         abbreviated `{Element+}`, represents a nonempty stream.

       Every iterator for a nonempty stream must produce at
       least one element.

       A value list in braces produces a new instance of
       `Iterable`:

           {String+} words = { "hello", "world" };

       An instance of `Iterable` may be iterated using a `for`
       loop:

           for (c in "hello world") { ... }

       Comprehensions provide a convenient syntax for
       transforming streams:

           {Integer+} lengths = { for (w in words) w.size };

       The `*.` operator may be used to evaluate an attribute
       or invoke a method of the elements of the stream,
       producing a new stream:

           {Integer+} lengths = words*.size;

       `Iterable` and its subtypes define various operations
       that return other iterable objects. Such operations come
       in two flavors:

       - _Lazy_ operations return a *view* of the receiving
         iterable object. If the underlying iterable object is
         mutable, then changes to the underlying object will be
         reflected in the resulting view. Lazy operations are
         usually efficient, avoiding memory allocation or
         iteration of the receiving iterable object.
       - _Eager_ operations return an immutable object. If the
         receiving iterable object is mutable, changes to this
         object will not be reflected in the resulting immutable
         object. Eager operations are often expensive, involving
         memory allocation and iteration of the receiving
         iterable object.

       Lazy operations are generally preferred, because they can
       be efficiently chained. For example:

           string.filter((c) => c.letter||c.digit)
                 .map(Character.uppercased)

       is much less expensive than:

           string.select((c) => c.letter||c.digit)
                 .collect(Character.uppercased)

       Furthermore, it is always easy to produce a new
       immutable iterable object given the view produced by a
       lazy operation. For example:

           [ *string.filter((c) => c.letter||c.digit)
                    .map(Character.uppercased) ]

       However, there are certain scenarios where an eager
       operation is more useful, more convenient, or no more
       expensive than a lazy operation, including:

       - sorting operations, for example [[sort]], which are
         eager by nature,
       - operations which result in a subset or subrange of the
         receiving stream, where structural sharing would or
         could result in unnecessary memory retention.

       Certain operations come in both lazy and eager flavors,
       for example:

       - [[map]] vs [[collect]],
       - [[filter]] vs [[select]],
       - [[List.sublist]] vs [[List.measure]].

       Lazy operations normally return an instance of `Iterable`,
       or even a [[List]], [[Map]], or [[Set]]. Eager operations
       usually return a [[sequence|Sequential]]. The method
       [[sequence]] materializes the current elements of a
       stream into a sequence.

       There is no meaningful generic definition of equality for
       streams. For some streams&mdash;for example,
       `List`s&mdash;order is significant; for others&mdash;for
       example, `Set`s&mdash;order is not significant. Therefore,
       unlike [[Collection]] and its subtypes, `Iterable` does
       not define nor require any form of
       [[value equality|Object.equals]], and some streams simply
       do not support value equality. It follows that the `==`
       operator should not be used to compare generic streams,
       unless the streams are known to share some additional
       structure.

       To compare the elements of two streams, taking order into
       account, use the function [[corresponding]].

           {Float*} xs = ... ;
           {Float*} ys = ... ;
           Boolean same = corresponding(xs, ys);"""
    see (`interface Collection`,
         `function corresponding`,
         `interface Iterator`)
    by ("Gavin")
    tagged("Streams")
    shared interface Iterable<out Element=Anything,
                              out Absent=Null>
            satisfies Category<>
            given Absent satisfies Null {

        "An iterator for the elements belonging to this stream.

         If this is a nonempty stream with type `{Element+}`,
         the iterator must produce at least one element."
        shared formal Iterator<Element> iterator();

        "Returns `true` if the iterator for this stream produces
         the given element, or `false` otherwise. In the case of
         an infinite stream, this operation might never terminate;
         furthermore, this default implementation iterates all
         the elements until found (or not), which might be very
         expensive."
        shared actual default Boolean contains(Object element)
                => any((e) => if (exists e) then e==element else false);

        "Determines if the stream is empty, that is to say, if
         the iterator returns no elements."
        shared default Boolean empty
                => iterator().next() is Finished;

        "The number of elements returned by the [[iterator]] of
         this stream, if the iterator terminates. In the case of
         an infinite stream, this operation never terminates."
        shared default Integer size => count((e) => true);

        "Determines if this stream has more elements than the
         given [[length]]. This is an efficient operation for
         streams with many elements."
        see (`value size`)
        shared default Boolean longerThan(Integer length) {
            if (length<0) {
                return true;
            }
            variable value count=0;
            for (element in this) {
                if (count++==length) {
                    return true;
                }
            }
            return false;
        }

        "Determines if this stream has fewer elements than the
         given [[length]]. This is an efficient operation for
         streams with many elements."
        see (`value size`)
        shared default Boolean shorterThan(Integer length) {
            if (length<=0) {
                return false;
            }
            variable value count=0;
            for (element in this) {
                if (++count==length) {
                    return false;
                }
            }
            return true;
        }

        "The first element returned by the iterator, if any, or
         `null` if this stream is empty. For a stream with an
         unstable iteration order, a different value might be
         produced each time `first` is evaluated."
        shared default Absent|Element first {
            if (!is Finished first = iterator().next()) {
                return first;
            }
            else {
                "iterator for nonempty iterable must produce at
                 least one element"
                assert (is Absent null);
                return null;
            }
        }

        "The last element returned by the iterator, if any, or
         `null` if this stream is empty. In the case of an
         infinite stream, this operation never terminates;
         furthermore, this default implementation iterates all
         elements, which might be very expensive."
        shared default Absent|Element last {
            variable Absent|Element e = first;
            for (x in this) {
                e = x;
            }
            return e;
        }

        "The [[index]]th element returned by an iterator of this
         stream, or `null` if there are fewer than `index+1`
         elements in the stream. For a stream with an unstable
         iteration order, a different value might be produced
         each time `getFromFirst(index)` is called for a given
         integer `index`."
        since("1.1.0")
        shared default Element? getFromFirst(Integer index) {
            variable value current = 0;
            for (element in this) {
                if (current++==index) {
                    return element;
                }
            }
            else {
                return null;
            }
        }

        "A [[sequence|Sequential]] containing all the elements
         of this stream, in the same order they occur in this
         stream. This operation eagerly evaluates and collects
         every element of the stream."
        since("1.1.0")
        shared default Element[] sequence()
                => let (array = Array(this))
                    if (array.empty)
                        then []
                        else ArraySequence(array);

        "A [[Range]] containing all indexes of this stream, or
         `[]` if this list is empty. The resulting range is
         equal to `0:size`."
        since("1.2.0")
        shared default Range<Integer>|[] indexes() => 0:size;

        "A stream containing all but the first element of this
         stream. For a stream with an unstable iteration order,
         a different stream might be produced each time `rest`
         is evaluated.

         Therefore, if the stream `i` has an unstable iteration
         order, the stream `{ i.first, *i.rest }` might not have
         the same elements as `i`."
        see (`value first`)
        shared default {Element*} rest => skip(1);

        "A stream containing all but the last element of this
         stream. For a stream with an unstable iteration order,
         a different stream might be produced each time
         `exceptLast` is evaluated."
        since("1.1.0")
        shared default {Element*} exceptLast
                => object satisfies {Element*} {
            iterator()
                    => let (iter = outer.iterator())
                object satisfies Iterator<Element> {
                    variable value current = iter.next();
                    shared actual Element|Finished next() {
                        if (!is Finished next = iter.next()) {
                            value result = current;
                            current = next;
                            return result;
                        }
                        else {
                            return finished;
                        }
                    }
                };
        };

        "Call the given [[function|step]] for each element of
         this stream, passing the elements in the order they
         occur in this stream.

         For example:

             words.each((word) {
                 print(word.lowercased);
                 print(word.uppercased);
             });

         Has the same effect as the following `for` loop:

             for (word in words) {
                 print(word.lowercased);
                 print(word.uppercased);
             }

         _For certain streams this method is highly efficient,
         surpassing the performance of `for` loops on the JVM.
         Thus, `each()` is sometimes preferred in highly
         performance-critical low-level code._"
        since("1.2.0")
        shared default void each(
            "The function to be called for each element in the
             stream."
            void step(Element element)) {
            for (element in this) {
                step(element);
            }
        }

        "Produces a stream containing the results of applying
         the given [[mapping|collecting]] to the elements of
         this stream.

         For any empty stream, `map()` returns an empty stream:

             {}.map(f) == {}

         For any nonempty stream `it`, and mapping function `f`,
         the result of `map()` may be obtained according to this
         recursive definition:

             it.map(f).first == f(it.first)
             it.map(f).rest == it.rest.map(f)

         Alternatively, and in practice, `map()` may be defined
         by this comprehension:

             it.map(f) == { for (e in it) f(e) }

         For example, the expression

             (0..4).map(10.power)

         results in the stream `{ 1, 10, 100, 1000, 10000 }`."
        see (`function collect`)
        shared default
        Iterable<Result,Absent> map<Result>(
            "The mapping to apply to the elements."
            Result collecting(Element element))
                => { for (elem in this) collecting(elem) };

        "Given a [[mapping function|collecting]] that accepts an
         [[Element]] and returns a stream of [[Result]]s,
         produces a new stream containing all elements of every
         `Result` stream that results from applying the function
         to the elements of this stream.

         For example, the expression

             { \"Hello\", \"World\" }.flatMap(String.lowercased)

         results in this stream:

             { 'h', 'e', 'l', 'l', 'o', 'w', 'o', 'r,' 'l', 'd' }

         The expression

             { \"hello\"->\"hola\", \"world\"->\"mundo\" }
                     .flatMap(Entry<String,String>.pair)

         produces this stream:

             { \"hello\", \"hola\", \"world\", \"mundo\" }"
        see (`function expand`)
        since("1.1.0")
        shared default
        Iterable<Result,Absent|OtherAbsent>
        flatMap<Result,OtherAbsent>(
            "The mapping function to apply to the elements of
             this stream, that produces a new stream of
             [[Result]]s."
            Iterable<Result,OtherAbsent> collecting(Element element))
                given OtherAbsent satisfies Null
                => expand(map(collecting));

        "Produces a stream containing the elements of this
         stream that satisfy the given [[predicate
         function|selecting]].

         For any empty stream, `filter()` returns an empty
         stream:

             {}.filter(p) == {}

         For any nonempty stream `it`, and predicate `p`, the
         result of `filter()` may be obtained according to this
         recursive definition:

             it.filter(p) == { if (p(it.first)) it.first }.chain(it.rest.filter(f))

         Alternatively, and in practice, `filter()` may be
         defined by this comprehension:

             it.filter(p) == { for (e in it) if (p(e)) e };

         For example, the expression

             (1..100).filter(13.divides)

         results in the stream `{ 13, 26, 39, 52, 65, 78, 91 }`."
        see (`function select`)
        shared default
        {Element*} filter(
            "The predicate the elements must satisfy. The
             elements which satisfy the predicate are included
             in the resulting stream."
            Boolean selecting(Element element))
                => { for (elem in this) if (selecting(elem)) elem };

        "Produces a stream containing the elements of this
         stream that are instances of the given [[type|Type]].

         For example, the expression

             { 1, 2, null, 3 }.narrow<Object>()

         results in the stream `{ 1, 2, 3 }` of type `{Integer*}`.

         If the type argument `Type` is not explicitly specified,
         [[Nothing]] is inferred, and the resulting stream is
         empty."
        since("1.2.0")
        shared default
        {Element&Type*} narrow<Type>()
                => { for (elem in this) if (is Type elem) elem };

        "Beginning with a given [[initial value|initial]], apply
         the given [[combining function|accumulating]] to each
         element of this stream in turn, progressively
         accumulating a single result.

         For an empty stream, `fold()` returns the given initial
         value `z`:

             {}.fold(z)(f) == z

         For a given nonempty stream `it`, initial value `z`,
         and combining function `f`, the result of `fold()` is
         obtained according to the following recursive
         definition:

             it.fold(z)(f) == f(it.exceptLast.fold(z)(f), it.last)

         For example, the expression

             (1..100).fold(0)(plus)

         results in the integer `5050`."
        see (`function reduce`,
             `function scan`)
        shared default
        Result fold<Result>(Result initial)(
            "The accumulating function that accepts an
             [[intermediate result|partial]], and the
             [[next element|element]]."
            Result accumulating(Result partial,
                                Element element)) {
            variable value partial = initial;
            for (elem in this) {
                partial = accumulating(partial, elem);
            }
            return partial;
        }

        "Beginning with the [[first]] element of this stream,
         apply the given [[combining function|accumulating]] to
         each element of this stream in turn, progressively
         accumulating a single result.

         For an empty stream, `reduce()` always returns `null`.

         For a stream with one element, `reduce()` returns that
         element:

             { first }.reduce(f) == first

         For a given stream `it` with more than one element, and
         combining function `f`, the result of `reduce()` is
         obtained according to the following recursive
         definition:

             it.reduce(f) == f(it.exceptLast.reduce(f), it.last)

         For example, the expression

             (1..100).reduce(plus)

         results in the integer `5050`."
        see (`function fold`)
        since("1.1.0")
        shared default
        Result|Element|Absent reduce<Result>(
            "The accumulating function that accepts an
             [[intermediate result|partial]], and the
             [[next element|element]]."
            Result accumulating(Result|Element partial,
                                Element element)) {
            value it = iterator();
            if (!is Finished initial = it.next()) {
                variable Result|Element partial = initial;
                while (!is Finished next = it.next()) {
                    partial = accumulating(partial, next);
                }
                return partial;
            }
            else {
                "iterable must be empty"
                assert (is Absent null);
                return null;
            }
        }

        "The stream of intermediate results obtained by
         beginning with a given [[initial value|initial]] and
         iteratively applying the given
         [[combining function|accumulating]] to each element of
         this stream in turn.

         For an empty stream, `scan()` returns a stream
         containing just the given initial value `z`:

             {}.scan(z)(f) == { z }

         For a given nonempty stream `it`, initial value `z`,
         and combining function `f`, the result of `scan()` is
         obtained according to the following recursive
         definition:

             it.scan(z)(f).last == f(it.exceptLast.scan(z)(f).last, it.last)
             it.scan(z)(f).exceptLast == it.exceptLast.scan(z)(f)

         The following identities explain the relationship
         between `scan` and [[fold]]:

             it.scan(z)(f).getFromFirst(n) == it.take(n).fold(z)(f)
             it.scan(z)(f).last == it.fold(z)(f)
             it.scan(z)(f).first == {}.fold(z)(f) == z

         For example, the expression

             (1..4).scan(0)(plus)

         results in the stream `{ 0, 1, 3, 6, 10 }`.

         This is a lazy operation and the resulting stream
         reflects changes to this stream."
        since("1.1.0")
        see (`function fold`)
        shared default
        {Result+} scan<Result>(Result initial)(
            "The accumulating function that accepts the
             [[running total|partial]] and the
             [[next element|element]]."
            Result accumulating(Result partial,
                                Element element))
                => object satisfies {Result+} {
            empty => false;
            first => initial;
            size => 1 + outer.size;
            iterator()
                    => let (iter = outer.iterator())
                object satisfies Iterator<Result> {
                    variable value returnInitial = true;
                    variable value partial = initial;
                    shared actual Result|Finished next() {
                        if (returnInitial) {
                            returnInitial = false;
                            return initial;
                        }
                        else if (!is Finished element = iter.next()) {
                            partial = accumulating(partial, element);
                            return partial;
                        }
                        else {
                            return finished;
                        }
                    }
                    string => outer.string + ".iterator()";
                };
        };

        "The first element of this stream which is not null and
         satisfies the [[given predicate function|selecting]],
         if any, or `null` if there is no such element. For an
         infinite stream, this method might not terminate.

         For example, the expression

             (-10..10).find(Integer.positive)

         evaluates to `1`."
        see (`function findLast`, `function locate`)
        shared default
        Element? find(
            "The predicate the element must satisfy."
            Boolean selecting(Element&Object element)) {
            for (elem in this) {
                if (exists elem, selecting(elem)) {
                    return elem;
                }
            }
            return null;
        }

        "The last element of this stream which is not null and
         satisfies the [[given predicate function|selecting]],
         if any, or `null` if there is no such element. For an
         infinite stream, this method will not terminate.

         For example, the expression

             (-10..10).findLast(3.divides)

         evaluates to `9`."
        see (`function find`, `function locateLast`)
        shared default
        Element? findLast(
            "The predicate the element must satisfy."
            Boolean selecting(Element&Object element)) {
            variable Element? last = null;
            for (elem in this) {
                if (exists elem, selecting(elem)) {
                    last = elem;
                }
            }
            return last;
        }

        "The first element of this stream which is not null and
         satisfies the [[given predicate function|selecting]],
         if any, together with its position in the stream, or
         `null` if there is no such element. For an infinite
         stream, this method might not terminate.

         For example, the expression

             (-10..10).locate(Integer.positive)

         evaluates to `11->1`."
        see (`function locateLast`, `function locations`,
             `function find`,
             `function List.firstIndexWhere`)
        since("1.2.0")
        shared default
        <Integer->Element&Object>? locate(
            "The predicate the element must satisfy."
            Boolean selecting(Element&Object element)) {
            variable value index = 0;
            for (elem in this) {
                if (exists elem, selecting(elem)) {
                    return index->elem;
                }
                index++;
            }
            return null;
        }

        "The last element of this stream which is not null and
         satisfies the [[given predicate function|selecting]],
         if any, together with its position in the stream, or
         `null` if there is no such element. For an infinite
         stream, this method might not terminate.

         For example, the expression

             (-10..10).locateLast(3.divides)

         evaluates to `19->9`."
        see (`function locate`, `function locations`,
             `function findLast`,
             `function List.lastIndexWhere`)
        since("1.2.0")
        shared default
        <Integer->Element&Object>? locateLast(
            "The predicate the element must satisfy."
            Boolean selecting(Element&Object element)) {
            variable <Integer->Element&Object>? last = null;
            variable value index = 0;
            for (elem in this) {
                if (exists elem, selecting(elem)) {
                    last = index->elem;
                }
                index++;
            }
            return last;
        }

        "A stream producing all elements of this stream which
         are not null and which satisfy the
         [[given predicate function|selecting]], together with
         their positions in the stream.

         For example, the expression

             (-5..5).locations(3.divides)

         evaluates to the stream `{ 2->-3, 5->0, 8->3 }`.

         Note that this method is more efficient than the
         alternative of applying [[filter]] to an [[indexed]]
         stream."
        see (`function locate`, `function locateLast`,
             `function List.indexesWhere`)
        since("1.2.0")
        shared default
        {<Integer->Element&Object>*} locations(
            "The predicate the element must satisfy."
            Boolean selecting(Element&Object element))
                => object satisfies {<Integer->Element&Object>*} {
            iterator()
                    => let (iter = outer.iterator())
                object satisfies Iterator<Integer->Element&Object> {
                    variable value i=0;
                    shared actual <Integer->Element&Object>|Finished next() {
                        while (!is Finished next = iter.next()) {
                            if (exists next, selecting(next)) {
                                return i++->next;
                            }
                            else {
                                i++;
                            }
                        }
                        return finished;
                    }
                    string => outer.string + ".iterator()";
                };
        };

        "Return the largest value in the stream, as measured by
         the given [[comparator function|comparing]] imposing a
         partial order upon the elements of the stream, or `null`
         if this stream is empty.

         For example, the expression

             {-10.0, -1.0, 5.0}.max(byIncreasing(Float.magnitude))

         evaluates to `-10`.

         For any nonempty stream `it`, and comparator function
         `c`, `it.max(c)` evaluates to the first element of `it`
         such that for every element `e` of `it`,
         `c(e, it.max(c)) != larger`.

         Note that the toplevel functions [[ceylon.language::max]]
         and [[ceylon.language::min]] may be used to find the
         largest and smallest values in a stream of [[Comparable]]
         values, according to the natural order of its elements."
        see (`function package.max`, `function package.min`,
             `function byIncreasing`, `function byDecreasing`,
             `function package.comparing`)
        since("1.1.0")
        shared default
        Element|Absent max(
            "The function comparing pairs of elements."
            Comparison comparing(Element x, Element y)) {
            value it = iterator();
            if (!is Finished first = it.next()) {
                variable value max = first;
                while (!is Finished val = it.next()) {
                    if (comparing(val,max)==larger) {
                        max = val;
                    }
                }
                return max;
            }
            else {
                "iterable must be empty"
                assert (is Absent null);
                return null;
            }
        }

        "Given a [[method]] of the element type [[Element]],
         return a function that, when supplied with a list of
         method arguments, produces a new iterable object that
         applies the `method` to each element of this iterable
         object in turn.

             {Boolean+}(Object) fun = (-1..1).spread(Object.equals);
             print(fun(0)); //prints { false, true, false }"
        since("1.1.0")
        shared default
        Iterable<Result,Absent>(*Args)
        spread<Result,Args>(Result(*Args) method(Element element))
                given Args satisfies Anything[]
                //=> flatten((Args args) => map(shuffle(method)(*args)));
                => flatten((Args args)
                    => { for (elem in this) method(elem)(*args) });

        /*"Produce a new [[sequence|Sequential]] containing the
         elements of this stream, in the reverse order to the
         order in which they occur in this stream.

         That is, if a stream `i` is stable, and if `x` and `y`
         are elements `i`, and `x` is produced before `y` by the
         [[iterator]] for `i`, then `y` occurs before `x` in the
         sequence `i.reverse()`.

         This operation is an eager counterpart to
         [[List.reversed]]."
        see (`value List.reversed`)
        shared default List<Element> reverse() {
            value array = Array(this);
            if (array.empty) {
                return [];
            }
            else {
                array.reverseInPlace();
                return ArraySequence(array);
            }
        }*/

        "Produce a new [[sequence|Sequential]] containing the
         elements of this stream, sorted according to the given
         [[comparator function|comparing]] imposing a partial
         order upon the elements of the stream.

         For convenience, the functions [[byIncreasing]] and
         [[byDecreasing]] produce suitable comparator functions.

         For example, this expression

             \"Hello World!\".sort(byIncreasing(Character.lowercased))

         evaluates to the sequence
         `[ , !, d, e, H, l, l, l, o, o, r, W].`

         This operation is eager by nature.

         Note that the toplevel function [[ceylon.language::sort]]
         may be used to sort a stream of [[Comparable]] values
         according to the natural order of its elements."
        see (`function increasing`, `function decreasing`,
             `function byIncreasing`, `function byDecreasing`,
             `function package.comparing`)
        shared default
        Element[] sort(
            "The function comparing pairs of elements."
            Comparison comparing(Element x, Element y)) {
            value array = Array(this);
            if (array.empty) {
                return [];
            }
            else {
                array.sortInPlace(comparing);
                return ArraySequence(array);
            }
        }

        "Produce a new [[sequence|Sequential]] containing the
         results of applying the given [[mapping|collecting]] to
         the elements of this stream.

         This operation is an eager counterpart to [[map]]. For
         any stream `it`, and mapping `f`:

             it.collect(f) == [*it.map(f)]"
        see (`function map`)
        shared default
        Result[] collect<Result>(
            "The transformation applied to the elements."
            Result collecting(Element element))
                => map(collecting).sequence();

        "Produce a new [[sequence|Sequential]] containing all
         elements of this stream that satisfy the given
         [[predicate function|selecting]], in the order in which
         they occur in this stream.

         This operation is an eager counterpart to [[filter]].
         For any stream `it`, and predicate `p`:

             it.select(p) == [*it.filter(p)]"
        see (`function filter`)
        shared default
        Element[] select(
            "The predicate the elements must satisfy."
            Boolean selecting(Element element))
                 => filter(selecting).sequence();

        "Produces the number of elements in this stream that
         satisfy the [[given predicate function|selecting]].
         For an infinite stream, this method never terminates."
        shared default
        Integer count(
            "The predicate satisfied by the elements to be
             counted."
            Boolean selecting(Element element)) {
            variable value count=0;
            for (elem in this) {
                if (selecting(elem)) {
                    count++;
                }
            }
            return count;
        }

        "Determines if there is at least one element of this
         stream that satisfies the given [[predicate
         function|selecting]]. If the stream is empty, returns
         `false`. For an infinite stream, this operation might
         not terminate."
        see (`function every`)
        shared default
        Boolean any(
            "The predicate that at least one element must
             satisfy."
            Boolean selecting(Element element)) {
            for (e in this) {
                if (selecting(e)) {
                    return true;
                }
            }
            return false;
        }

        "Determines if all elements of this stream satisfy the
         given [[predicate function|selecting]]. If the stream
         is empty, return `true`. For an infinite stream, this
         operation might not terminate."
        see (`function any`)
        shared default
        Boolean every(
            "The predicate that all elements must satisfy."
            Boolean selecting(Element element)) {
            for (e in this) {
                if (!selecting(e)) {
                    return false;
                }
            }
            return true;
        }

        "Produces a stream containing the elements of this
         stream, after skipping the first [[skipping]] elements
         produced by its iterator.

         If this stream does not contain more elements than the
         specified number of elements to skip, the resulting
         stream has no elements. If the specified number of
         elements to skip is zero or fewer, the resulting stream
         contains the same elements as this stream."
        see (`function List.sublistFrom`,
             `function skipWhile`,
             `function take`)
        since("1.1.0")
        shared default
        {Element*} skip(Integer skipping) {
            if (skipping <= 0) {
                return this;
            }
            else {
                return object satisfies {Element*} {
                    shared actual Iterator<Element> iterator() {
                        value iter = outer.iterator();
                        variable value i=0;
                        while (i++<skipping &&
                                !iter.next() is Finished) {}
                        return iter;
                    }
                    string => outer.string + ".iterator()";
                };
            }
        }

        "Produces a stream containing the first [[taking]]
         elements of this stream.

         If the specified number of elements to take is larger
         than the number of elements of this stream, the
         resulting stream contains the same elements as this
         stream. If the specified number of elements to take is
         fewer than one, the resulting stream has no elements."
        see (`function List.sublistTo`,
             `function List.initial`,
             `function takeWhile`,
             `function skip`)
        since("1.1.0")
        shared default
        {Element*} take(Integer taking) {
            if (taking <= 0) {
                return {};
            }
            else {
                return object satisfies {Element*} {
                    shared actual Iterator<Element> iterator() {
                        value iter = outer.iterator();
                        return object satisfies Iterator<Element> {
                            variable value i=0;
                            next() => ++i>taking
                                        then finished
                                        else iter.next();
                            string => outer.string + ".iterator()";
                        };
                    }
                    first => outer.first;
                };
            }
        }

        "Produces a stream containing the elements of this
         stream, after skipping the leading elements until the
         given [[predicate function|skipping]] returns `false`."
        see (`function skip`,
             `function takeWhile`)
        since("1.1.0")
        shared default
        {Element*} skipWhile(
            "The function that returns `false` when the
             resulting stream should stop skipping elements from
             the stream."
            Boolean skipping(Element element))
                => object satisfies {Element*} {
            shared actual Iterator<Element> iterator() {
                value iter = outer.iterator();
                while (!is Finished elem = iter.next()) {
                    if (!skipping(elem)) {
                        return object satisfies Iterator<Element> {
                            variable Boolean first=true;
                            actual shared Element|Finished next() {
                                if (first) {
                                    first = false;
                                    return elem;
                                }
                                else {
                                    return iter.next();
                                }
                            }
                            string => outer.string + ".iterator()";
                        };
                    }
                }
                return emptyIterator;
            }
        };

        "Produces a stream containing the leading elements of
         this stream until the given [[predicate function|taking]]
         returns `false`."
        see (`function take`,
             `function skipWhile`)
        since("1.1.0")
        shared default
        {Element*} takeWhile(
            "The function that returns `false` when the
             resulting stream should stop taking elements from
             this stream."
            Boolean taking(Element element))
                => object satisfies {Element*} {
            iterator()
                    => let (iter = outer.iterator())
                object satisfies Iterator<Element> {
                    variable Boolean alive = true;
                    actual shared Element|Finished next() {
                        if (alive,
                            !is Finished next = iter.next()) {
                            if (taking(next)) {
                                return next;
                            }
                            else {
                                alive = false;
                            }
                        }
                        return finished;
                    }
                    string => outer.string + ".iterator()";
                };
        };

        "Produces a stream formed by repeating the elements of
         this stream the given [[number of times|times]], or an
         empty stream if `times<=0`.

         For example, the expression

             { 1, 2 }.repeat(3)

         evaluates to the stream `{ 1, 2, 1, 2, 1, 2 }`.

         If this is a stream with an unstable iteration order,
         the elements of the resulting stream do not occur in
         repeating order.

         This is a lazy operation and the resulting stream
         reflects changes to this stream."
        see (`value cycled`)
        shared default
        {Element*} repeat(Integer times)
                => object satisfies {Element*} {
            size => times * outer.size;
            string => "(``outer.string``).repeat(``times``)";
            iterator() => CycledIterator(outer,times);
        };

        "Produces a stream containing every [[step]]th element
         of this stream. If the step size is `1`, the resulting
         stream contains the same elements as this stream.

         For example, the expression

             (0..10).by(3)

         results in the stream `{ 0, 3, 6, 9 }`.

         The step size must be greater than zero."
        throws (`class AssertionError`,
                "if the given step size is nonpositive,
                 i.e. `step<1`")
        shared default
        Iterable<Element,Absent> by(Integer step) {
            "step size must be greater than zero"
            assert (step > 0);
            if (step == 1) {
                return this;
            }
            else {
                return object satisfies Iterable<Element,Absent> {
                    string => "(``outer.string``).by(``step``)";
                    iterator()
                            => let (iter = outer.iterator())
                        object satisfies Iterator<Element> {
                            shared actual Element|Finished next() {
                                value next = iter.next();
                                variable value i=0;
                                while (++i<step &&
                                    !iter.next() is Finished) {}
                                return next;
                            }
                            string => outer.string + ".iterator()";
                        };
                };
            }
        }

        "Produces a stream containing the elements of this
         stream, in the order in which they occur in this stream,
         after replacing every `null` element in the stream with
         the [[given default value|defaultValue]]. The value
         `null` does not ocur in the resulting stream.

         For example, the expression

             { \"123\", \"abc\", \"456\" }.map(parseInteger).defaultNullElements(0)

         results in the stream `{ 123, 0, 456 }`."
        see (`value coalesced`)
        shared default
        Iterable<Element&Object|Default,Absent>
        defaultNullElements<Default>(
            "A default value that replaces `null` elements."
            Default defaultValue)
                given Default satisfies Object
                => map((elem) => elem else defaultValue);

        "The non-null elements of this stream, in the order in
         which they occur in this stream. For null elements of
         the original stream, there is no entry in the resulting
         stream.

         For example, the expression

             { \"123\", \"abc\", \"456\"}.map(parseInteger).coalesced

         results in the stream `{ 123, 456 }`."
        see (`function defaultNullElements`)
        shared default
        {Element&Object*} coalesced
                => { for (e in this) if (exists e) e };

        "A stream containing all [[entries|Entry]] of form
         `index->element` where `element` is an element of this
         stream, and `index` is the position at which `element`
         occurs in this stream, ordered by increasing `index`.

         For example, the expression

             { \"hello\", null, \"world\" }.indexed

         results in the stream `{ 0->\"hello\", 1->null, 2->\"world\" }`."
        see (`function locations`)
        shared default
        Iterable<<Integer->Element>,Absent> indexed
                => object
                satisfies Iterable<<Integer->Element>,Absent> {
            iterator()
                    => let (iter = outer.iterator())
                object satisfies Iterator<Integer->Element> {
                    variable value i=0;
                    next() => switch (next = iter.next())
                              case (finished) finished
                              else i++ -> next;
                    string => outer.string + ".iterator()";
                };
        };

        "A stream containing whose elements are pairs (2-tuples)
         comprising an element of this stream paired with the
         next element in the stream. The resulting stream has
         one fewer elements than this stream. If this stream has
         exactly one element, the resulting stream is empty.

         For example, the expression

             (1..5).paired

         results in the stream
         `{ [1, 2], [2, 3], [3, 4], [4, 5] }`.

         This expression determines if a stream is monotonically
         increasing:

             every { for ([x, y] in nums.paired) x < y }

         For any stable `stream`, this operation is equivalent
         to `zipPairs(stream,stream.rest)`.

         _If this is a stream with an unstable iteration order,
         the resulting stream produces a different set of pairs
         each time it is iterated, thus violating the general
         contract for an immutable finite stream._

         This is a lazy operation and the resulting stream
         reflects changes to this stream."
        since("1.1.0")
        shared default
        {Element[2]*} paired
                => object satisfies {Element[2]*} {
            size => let (size = outer.size-1)
                    if (size<0) then 0 else size;
            empty => outer.size<2;
            iterator()
                    => let (iter = outer.iterator())
                object satisfies Iterator<Element[2]> {
                    variable value previous = iter.next();
                    shared actual Element[2]|Finished next() {
                        if (!is Finished head = previous,
                            !is Finished tip = iter.next()) {
                            previous = tip;
                            return [head, tip];
                        }
                        else {
                            return finished;
                        }
                    }
                };
        };

        "Produces a stream of sequences of the given [[length]],
         containing elements of this stream. Each sequence in
         the stream contains the next [[length]] elements of
         this sequence that have not yet been assigned to a
         previous sequence, in the same order that they occur in
         this stream. The very last sequence in the stream may
         be shorter than the given `length`.

         For example, the expression

             \"hello\".partition(2)

         results in the stream `{ ['h','e'], ['l','l'], ['o'] }.`

         For any `stream` and for any strictly positive integer
         [[length]]:

             expand { stream.partition(length) } == stream

         _If this is a stream with an unstable iteration order,
         the resulting stream produces a different set of pairs
         each time it is iterated, thus violating the general
         contract for an immutable finite stream._

         This is a lazy operation and the resulting stream
         reflects changes to this stream."
        throws (`class AssertionError`,
                "if `length<=0`")
        since("1.1.0")
        shared default
        Iterable<[Element+],Absent> partition(
            "The length of the sequences in the resulting stream,
             which must be strictly positive."
            Integer length) {
            "length must be strictly positive"
            assert (length>0);
            return object satisfies Iterable<[Element+],Absent> {
                size => let (outerSize = outer.size,
                              quotient = outerSize/length)
                            if (length.divides(outerSize))
                                then quotient
                                else quotient+1;
                empty => outer.empty;
                iterator()
                        => let (iter = outer.iterator())
                    object satisfies Iterator<[Element+]> {
                        shared actual [Element+]|Finished next() {
                            if (!is Finished next = iter.next()) {
                                value array =
                                        Array.ofSize(length, next);
                                variable value index = 0;
                                while (++index<length) {
                                    if (!is Finished current
                                            = iter.next()) {
                                        array.set(index, current);
                                    }
                                    else {
                                        return ArraySequence(
                                            array[...index-1]);
                                    }
                                }
                                return ArraySequence(array);
                            }
                            else {
                                return finished;
                            }
                        }
                    };
            };
        }

        "Produces a stream with a given [[initial element|head]],
         followed by the elements of this stream, in the order
         in which they occur in this stream.

         For example, the expression

             (1..3).follow(0)

         evaluates to the stream `{ 0, 1, 2, 3 }`.

         Note that the expression `stream.follow(head)` eagerly
         evaluates `head`, and therefore is not precisely the
         same as this enumeration expression, where `head` is
         evaluated lazily:

             { head, *stream }"
        see (`function chain`)
        since("1.1.0")
        shared default
        {Element|Other+} follow<Other>(Other head)
                => { head, *this };

        "The elements of this stream, in the order in which they
         occur in this stream, followed by the elements of the
         [[given stream|other]] in the order in which they occur
         in the given stream.

         For example, the expression

             (1..3).chain(\"abc\")

         evaluates to the stream `{ 1, 2, 3, 'a', 'b', 'c' }`."
        see (`function expand`)
        shared default
        Iterable<Element|Other,Absent&OtherAbsent>
        chain<Other,OtherAbsent>
            (Iterable<Other,OtherAbsent> other)
                given OtherAbsent satisfies Null
                => object
                satisfies Iterable<Element|Other,
                                   Absent&OtherAbsent> {
            iterator() => ChainedIterator(outer, other);
        };

        "A stream of pairs of elements of this stream and the
         the given stream, where for each element `x` of this
         stream, and element `y` of the given stream, the pair
         `[x,y]` belongs to the resulting stream. The pairs are
         sorted first by the position of `x` in this stream, and
         then by the position of `y` in the given stream.

         For example, this expression

             (1..3).product(\"ab\")

         evaluates to the stream
         `{ [1,'a'], [1,'b'], [2,'a'], [2,'b'], [3,'a'], [3,'b'] }`."
        since("1.1.0")
        shared default
        Iterable<[Element,Other],Absent|OtherAbsent>
        product<Other,OtherAbsent>
            (Iterable<Other,OtherAbsent> other)
                given OtherAbsent satisfies Null
                => { for (x in this) for (y in other) [x,y] };

        "An infinite stream that produces the elements of this
         stream, repeatedly.

         For example, the expression

             {6, 9}.cycled.take(5)

         evaluates to the stream `{ 6, 9, 6, 9, 6 }`.

         If this stream is empty, the resulting stream also
         empty."
        see (`function repeat`)
        shared default
        Iterable<Element,Absent> cycled
                => object satisfies Iterable<Element,Absent> {
            value orig => outer;
            string => "(``outer.string``).cycled";
            shared actual Integer size {
                "stream is infinite"
                assert (false);
            }
            iterator()
                    => object satisfies Iterator<Element> {
                    variable Iterator<Element> iter
                            = emptyIterator;
                    shared actual Element|Finished next() {
                        if (!is Finished next = iter.next()) {
                            return next;
                        }
                        else {
                            iter = orig.iterator();
                            return iter.next();
                        }
                    }
                    string => outer.string + ".iterator()";
                };
        };

        "A stream that contains the given [[element]] interposed
         between blocks of [[step]] elements of this stream. The
         resulting stream starts with the [[first]] element of
         this stream and ends with the [[last]] element of this
         stream. Elements of this stream occur in the resulting
         stream in the same order they occur in this stream.

         For example, the expression

             String(\"hello\".interpose(' '))

         evaluates to the string `\"h e l l o\"`.

         This is a lazy operation and the resulting stream
         reflects changes to this stream."
        throws (`class AssertionError`, "if `step<1`")
        see (`function interleave`)
        since("1.1.0")
        shared default
        Iterable<Element|Other,Absent> interpose<Other>(
            "The value to interpose between blocks of
             elements of this stream."
            Other element,
            "The step size that determines how often the
             given [[element]] occurs in the resulting
             stream. The `element` occurs after each block
             of size `step` of elements of this stream. If
             `step==1`, the `element` occurs at every second
             position. The step size must be strictly positive."
            Integer step=1) {
            "step must be strictly positive"
            assert (step>=1);
            return object
                    satisfies Iterable<Element|Other,Absent> {
                shared actual Integer size {
                    value outerSize = outer.size;
                    return if (outerSize>0)
                        then outerSize + (outerSize-1)/step
                        else 0;
                }
                empty => outer.empty;
                first => outer.first;
                last => outer.last;
                iterator()
                        => let (iter = outer.iterator())
                    object satisfies Iterator<Element|Other> {
                        variable value current = iter.next();
                        variable value count = 0;
                        shared actual Element|Other|Finished next() {
                            if (!is Finished curr = current) {
                                if ((step+1).divides(++count)) {
                                    return element;
                                }
                                else {
                                    current = iter.next();
                                    return curr;
                                }
                            }
                            else {
                                return finished;
                            }
                        }
                        string => outer.string + ".iterator()";
                    };
            };
        }

        "A stream that produces every element produced by this
         stream exactly once. Duplicate elements of this stream
         are eliminated. Two elements are considered distinct
         unless they are both [[null|Null]], or unless they are
         both non-null and [[equal|Object.equals]].

         For example:

             String(\"hello world\".distinct)

         is the string `\"helo wrd\"`.

         This is a lazy operation and the resulting stream
         reflects changes to this stream."
        see(`function set`)
        since("1.2.0")
        shared default Iterable<Element,Absent> distinct
                => object satisfies Iterable<Element,Absent> {
            iterator()
                    => let (elements=outer)
                    object satisfies Iterator<Element> {

                alias MaybeEntry => ElementEntry<Element>?;

                value it = elements.iterator();
                variable value count = 0;

                variable value store
                        = Array.ofSize {
                    size = 16;
                    element = null of MaybeEntry;
                };

                function hash(Element element, Integer size)
                        => if (exists element)
                        then element.hash.magnitude % size
                        else 0;

                function rebuild(Array<MaybeEntry> store) {
                    value newStore
                            = Array.ofSize {
                        size = store.size*2;
                        element = null of MaybeEntry;
                    };
                    for (entries in store) {
                        variable value entry = entries;
                        while (exists e = entry) {
                            value index = hash {
                                element = e.element;
                                size = newStore.size;
                            };
                            newStore.set(index,
                                ElementEntry {
                                    next = newStore[index];
                                    element = e.element;
                                });
                            entry = e.next;
                        }
                    }
                    return newStore;
                }

                shared actual Element|Finished next() {
                    while (true) {
                        switch (element = it.next())
                        case (is Finished) {
                            return element;
                        }
                        else {
                            value index = hash {
                                element = element;
                                size = store.size;
                            };
                            value entry = store[index];
                            if (exists entry, entry.has(element)) {
                                //keep iterating
                            }
                            else {
                                store.set(index,
                                    ElementEntry {
                                        next = entry;
                                        element = element;
                                    });
                                count++;
                                if (count>store.size*2) {
                                    store = rebuild(store);
                                }
                                return element;
                            }
                        }
                    }
                }
            };
        };

        "Produce a [[Map]] mapping elements to frequencies where
         each [[entry|Entry]] maps a distinct non-null element
         of this stream to the number of times the element was
         produced by the stream. Elements are considered
         distinct if they are not [[equal|Object.equals]]. Null
         elements of this stream are simply discarded.

         For example:

             \"helloworld\".frequencies()

         produces the map
         `{ r->1, d->1, e->1, w->1, h->1, l->3, o->2 }`.

         This is an eager operation, and the resulting map does
         not reflect changes to this stream."
        since("1.2.0")
        shared Map<Element&Object,Integer> frequencies()
                => coalesced.summarize(identity,
                        (Integer? count, _)
                                => (count else 0) + 1);

        "Produces a [[Map]] mapping elements to items where each
         [[entry|Entry]] maps a distinct non-null element of
         this stream to the item produced by the given
         [[function|collecting]]. Elements are considered
         distinct if they are not [[equal|Object.equals]]. Null
         elements of this stream are simply discarded.

         For example:

             (1..5).tabulate(2.divides)

         produces the map
         `{ 1->false, 2->true, 3->false, 4->true, 5->false }`.

         This is an eager operation, and the resulting map does
         not reflect changes to this stream."
        since("1.2.0")
        shared Map<Element&Object,Result> tabulate<Result>(
            "A function that produces an item for the given
             [[key]], an element of this stream."
            Result collecting(Element key))
                => coalesced.summarize(identity,
                        (Result? item, key)
                                => if (exists item)
                                then item else collecting(key));

        "Classifies the elements of this stream into a new
         immutable [[Map]] where each key is a value produced by
         the given [[grouping function|grouping]] and each
         corresponding item is [[sequence|Sequence]] of all
         elements that produced the key when passed as arguments
         to the grouping function.

         Within each group, the sequence elements occur in the
         same order they occurred in this stream.

         For example:

             (0..10).group((i) => i.even then \"even\" else \"odd\")

         produces the map
         `{ even->[0, 2, 4, 6, 8, 10], odd->[1, 3, 5, 7, 9] }`.

         This is an eager operation, and the resulting map does
         not reflect changes to this stream."
        see(`function summarize`)
        since("1.2.0")
        shared Map<Group,[Element+]> group<Group>(
            "The grouping function that assigns a key to the
             given [[element]]. Multiple elements may be
             assigned to the same key, indicating that they
             belong to the same [[Group]] in the resulting map."
            Group grouping(Element element))
                given Group satisfies Object
                => summarize<Group,ElementEntry<Element>>
                        (grouping, ElementEntry)
                   .mapItems((_, item) => item.reversedSequence());

        "Efficiently [[group]] and [[fold]] the elements of this
         stream in a single step.

         For example, the expression:

             (1..10)
                .summarize((i) => i%3,
                    (Integer[2]? pair, i)
                        => if (exists [sum, product] = pair)
                           then [sum+i, product*i] else [i,i])

         produces the map
         `{ 0->[18, 162], 1->[22, 280], 2->[15, 80] }`, being
         equivalent to, but much more efficient than, the
         following expression written using `group()`,
         `mapItems()` and `fold()`:

             (1..10)
                 .group((i) => i%3)
                 .mapItems((_, item)
                    => item.fold([0,1])
                        (([sum, product], i)
                            => [sum+i, product*i]))

         This is an eager operation, and the resulting map does
         not reflect changes to this stream."
        see(`function group`, `function fold`)
        since("1.2.0")
        shared Map<Group,Result> summarize<Group,Result>(
            "The grouping function that assigns a key to the
             given [[element]]. Multiple elements may be
             assigned to the same key, indicating that they
             should be aggregated by calling [[accumulating]]."
            Group grouping(Element element),
            "The accumulating function that accepts an
             [[intermediate result|partial]] for a key, and the
             [[next element]] with that key."
            Result accumulating(Result? partial, Element element))
                given Group satisfies Object
                => Summary(this, grouping, accumulating);

        "A string of form `\"{ x, y, z }\"` where `x`, `y`, and
         `z` are the `string` representations of the elements of
         this collection, as produced by the iterator of the
         stream, or the string `\"{}\"` if this stream is empty.
         If the stream is very long, the list of elements might
         be truncated, as indicated by an ellipse."
        shared actual default String string
                => let (elements = take(31).sequence())
                if (elements.empty) then
                    "{}"
                else if (elements.size==31) then
                    "{ ``commaList(elements.take(30))``, ... }"
                else
                    "{ ``commaList(elements)`` }";

    }

    String commaList({Anything*} elements)
            => ", ".join { for (e in elements) stringify(e) };

    class ElementEntry<Element>(next, element) {

        shared Element element;
        shared ElementEntry<Element>? next;

        shared Element first => element;

        shared Boolean has(Anything element) {
            variable ElementEntry<Element>? entry = this;
            while (exists e = entry) {
                if (exists element) {
                    if (exists ee = e.element,
                        element == ee) {
                        return true;
                    }
                }
                else {
                    if (!e.element exists) {
                        return true;
                    }
                }
                entry = e.next;
            }
            return false;
        }

        shared Integer size {
            variable value count = 1;
            variable value entry = this;
            while (exists next = entry.next) {
                entry = next;
                count++;
            }
            return count;
        }

        shared [Element+] reversedSequence() {
            //TODO: give Array a special-purpose
            //      constructor for this
            value size = this.size;
            value array = Array.ofSize {
                size = size;
                element = first;
            };
            variable value i = size;
            variable ElementEntry<Element>? entry = this;
            while (exists next = entry) {
                array.set(--i, next.element);
                entry = next.next;
            }
            return ArraySequence(array);
        }
    }

    class GroupEntry<Group,Result>(next, group, elements)
            given Group satisfies Object {

        shared Group group;
        shared variable Result elements;
        shared GroupEntry<Group,Result>? next;

        shared GroupEntry<Group,Result>? get(Object group) {
            variable GroupEntry<Group,Result>? entry = this;
            while (exists e = entry) {
                if (group == e.group) {
                    return e;
                }
                entry = e.next;
            }
            return null;
        }

    }

    see(`function Iterable.summarize`)
    class Summary<Element,Group,Result>(
        {Element*} elements,
        Group grouping(Element element),
        Result accumulating(Result? partial, Element element))
                extends Object() satisfies Map<Group,Result>
                given Group satisfies Object {

        alias MaybeEntry => GroupEntry<Group,Result>?;

        variable value store
                = Array.ofSize {
            size = 16;
            element = null of MaybeEntry;
        };

        function hash(Object group, Integer size)
                => group.hash.magnitude % size;

        function rebuild(Array<MaybeEntry> store) {
            value newStore
                    = Array.ofSize {
                size = store.size*2;
                element = null of MaybeEntry;
            };
            for (groups in store) {
                variable value group = groups;
                while (exists g = group) {
                    value index = hash {
                        group = g.group;
                        size = newStore.size;
                    };
                    newStore.set(index,
                        GroupEntry {
                            next = newStore[index];
                            group = g.group;
                            elements = g.elements;
                        });
                    group = g.next;
                }
            }
            return newStore;
        }

        variable value count = 0;
        for (element in elements) {
            value group = grouping(element);
            value index = hash {
                group = group;
                size = store.size;
            };
            value entries = store[index];
            if (exists entries,
                exists entry = entries.get(group)) {
                entry.elements = accumulating {
                    partial = entry.elements;
                    element = element;
                };
                //keep iterating
            }
            else {
                store.set(index,
                    GroupEntry {
                        next = entries;
                        group = group;
                        elements = accumulating {
                            partial = null;
                            element = element;
                        };
                    });
                count++;
                if (count>store.size*2) {
                    store = rebuild(store);
                }
            }
        }

        size => count;

        iterator()
                => object satisfies Iterator<Group->Result> {
            variable value index = 0;
            variable GroupEntry<Group,Result>? entry = null;
            shared actual <Group->Result>|Finished next() {
                GroupEntry<Group,Result> result;
                if (exists e = entry) {
                    entry = e.next;
                    result = e;
                }
                else {
                    while (true) {
                        if (index>=store.size) {
                            return finished;
                        }
                        else {
                            entry = store[index++];
                            if (exists e = entry) {
                                entry = e.next;
                                result = e;
                                break;
                            }
                        }
                    }
                }
                return result.group -> result.elements;
            }
        };

        clone() => this;

        function group(Object key)
                => store[hash(key, store.size)]?.get(key);

        defines(Object key) => group(key) exists;

        get(Object key) => group(key)?.elements;

        shared actual Result|Default getOrDefault<Default>
                (Object key, Default default)
                => if (exists group = group(key))
                then group.elements else default;

    }