    """A _tuple_ is a typed linked list. Each instance of
       `Tuple` represents the value and type of a single link.
       The attributes `first` and `rest` allow us to retrieve a
       value from the list without losing its static type
       information.

           value point = Tuple(0.0, Tuple(0.0, Tuple("origin", [])));
           Float x = point.first;
           Float y = point.rest.first;
           String label = point.rest.rest.first;

       Usually, we abbreviate code involving tuples.

           [Float,Float,String] point = [0.0, 0.0, "origin"];
           Float x = point[0];
           Float y = point[1];
           String label = point[2];

       A list of types enclosed in brackets is an abbreviated
       tuple type. An instance of `Tuple` may be constructed by
       surrounding a value list in brackets:

           [String,String] words = ["hello", "world"];

       The index operator with a literal integer argument is a
       shortcut for a chain of evaluations of `rest` and
       `first`. For example, `point[1]` means `point.rest.first`.

       A _terminated_ tuple type is a tuple where the type of
       the last link in the chain is `Empty`. An _unterminated_
       tuple type is a tuple where the type of the last link
       in the chain is `Sequence` or `Sequential`. Thus, a
       terminated tuple type has a length that is known
       statically. For an unterminated tuple type only a lower
       bound on its length is known statically.

       Here, `point` is an unterminated tuple:

           String[] labels = ... ;
           [Float,Float,String*] point = [0.0, 0.0, *labels];
           Float x = point[0];
           Float y = point[1];
           String? firstLabel = point[2];
           String[] allLabels = point[2...];"""
    by ("Gavin")
    tagged("Sequences", "Basic types", "Collections")
    shared final serializable native
    class Tuple<out Element, out First, out Rest = []>
            (first, rest)
            extends Object()
            satisfies [Element+]
            given First satisfies Element
            given Rest satisfies Element[] {

        "The first element of this tuple. (The head of the
         linked list.)"
        shared actual native
        First first;

        "A tuple with the elements of this tuple, except for the
         first element. (The tail of the linked list.)"
        shared actual native
        Rest rest;

        shared actual native
        Integer lastIndex => rest.size;

        shared actual native
        Integer size => 1 + rest.size;

        shared actual native
        Element? getFromFirst(Integer index)
                => switch (index <=> 0)
                case (smaller) null
                case (equal) first
                case (larger) rest.getFromFirst(index - 1);

        "The last element of this tuple."
        shared actual native
        Element last
                => if (nonempty rest)
                then rest.last
                else first;

        shared actual native
        Element[] measure(Integer from, Integer length) {
            if (length <= 0) {
                return [];
            }
            Integer realFrom = from < 0 then 0 else from;
            if (realFrom == 0) {
                return length == 1
                        then [first]
                        else rest[0 : length + realFrom - 1]
                                .withLeading(first);
            }
            return rest[realFrom - 1 : length];
        }

        shared actual native
        Element[] span(Integer from, Integer end) {
            if (from < 0 && end < 0) { return []; }
            Integer realFrom = from < 0 then 0 else from;
            Integer realEnd = end < 0 then 0 else end;
            return realFrom <= realEnd
                then this[from : realEnd - realFrom + 1]
                else this[realEnd : realFrom - realEnd + 1]
                            .reversed.sequence();
        }

        shared actual native
        Element[] spanTo(Integer to)
                => to < 0 then [] else span(0, to);

        shared actual native
        Element[] spanFrom(Integer from)
                => span(from, size);

        "This tuple."
        shared actual native
        Tuple<Element,First,Rest> clone() => this;

        shared actual native
        Iterator<Element> iterator()
                => object
                satisfies Iterator<Element> {
            variable Element[] current = outer;
            shared actual Element|Finished next() {
                if (nonempty c = current) {
                    current = c.rest;
                    return c.first;
                }
                else {
                    return finished;
                }
            }
            string => "``outer.string``.iterator()";
        };

        "Determine if the given value is an element of this
         tuple."
        shared actual native
        Boolean contains(Object element)
                => if (exists first, first == element)
                then true
                else element in rest;

        "Return a new tuple that starts with the specified
         [[element]], followed by the elements of this tuple."
        shared actual native
        Tuple<Element|Other,Other,Tuple<Element,First,Rest>>
        withLeading<Other>(
                "The first element of the resulting tuple."
                Other element)
                => Tuple(element, this);

        "Return a new tuple containing the elements of this
         tuple, followed by the given [[element]]."
        shared actual native
        [First,Element|Other+] withTrailing<Other>(
                "The last element of the resulting tuple."
                Other element)
                => Tuple(first, rest.withTrailing(element));

        "Return a tuple containing the elements of this
         tuple, followed by the given [[elements]]."
        shared actual native
        [First,Element|Other*] append<Other>(
                "The list of elements to be appended."
                Other[] elements)
                => Tuple(first, rest.append(elements));
    }