    "A [[Range]] of adjacent [[Enumerable]] values generated by
     a [[first]] element, and a strictly positive [[size]]. The
     range includes all values whose offset from `first` is
     non-negative and less than the `size`."
    see (`class Span`,
        `interface Enumerable`)
    final serializable
    class Measure<Element>(first, size)
            extends Range<Element>()
            given Element satisfies Enumerable<Element> {

        "The start of the range."
        shared actual Element first;

        "The size of the range."
        shared actual Integer size;

        "must be nonempty"
        assert (size > 0);

        string => first.string + ":" + size.string;

        last => first.neighbour(size - 1);

        longerThan(Integer length) => size > length;

        shorterThan(Integer length) => size < length;

        lastIndex => size - 1;

        rest => size > 1
                then Measure(first.successor, size - 1)
                else [];

        getFromFirst(Integer index)
                => index >= 0 && index < size
                then first.neighbour(index)
                else null;

        increasing => true;
        decreasing => false;

        iterator()
                => object satisfies Iterator<Element> {
            variable value count = 0;
            variable value current = first;
            shared actual Element|Finished next() {
                if (count >= size) {
                    return finished;
                }
                else if (count++ == 0) {
                    return current;
                }
                else {
                    return ++current;
                }
            }
            string => "(``outer.string``).iterator()";
        };

        shared actual
        {Element+} by(Integer step) {
            "step size must be greater than zero"
            assert (step > 0);
            return step == 1 then this else By(step);
        }

        class By(Integer step)
                satisfies {Element+} {

            size => 1 + (outer.size - 1) / step;

            first => outer.first;

            string => "(``outer``).by(``step``)";

            iterator() => object
                    satisfies Iterator<Element> {
                variable value count = 0;
                variable value current = first;
                shared actual Element|Finished next() {
                    if (count >= size) {
                        return finished;
                    }
                    else if (count++ == 0) {
                        return current;
                    }
                    else {
                        current = current.neighbour(step);
                        return current;
                    }
                }
                string => "``outer``.iterator()";
            };
        }

        shifted(Integer shift)
                => shift == 0
                then this
                else Measure(first.neighbour(shift), size);

        containsElement(Element x)
                => 0 <= x.offset(first) < size;

        shared actual
        Boolean includesRange(Range<Element> range) {
            switch (range)
            case (is Measure<Element>) {
                value offset = range.first.offset(first);
                return 0 <= offset <= size - range.size;
            }
            case (is Span<Element>) {
                if (range.decreasing) {
                    return false;
                } else {
                    value offset = range.first.offset(first);
                    return 0 <= offset <= size - range.size;
                }
            }
        }

        shared actual
        Boolean equals(Object that) {
            if (is Measure<out Object> that) {
                //optimize for another Measure
                return that.size == size && that.first == first;
            } else if (is Span<out Object> that) {
                return that.increasing &&
                        that.first == first && that.size == size;
            } else {
                //it might be another sort of List
                return super.equals(that);
            }
        }

        shared actual
        Element[] measure(Integer from, Integer length) {
            if (length <= 0) {
                return [];
            } else {
                value len = from + length < size
                        then length
                        else size - from;
                return Measure(first.neighbour(from), len);
            }
        }

        shared actual
        Element[] span(Integer from, Integer to) {
            if (from <= to) {
                if (to < 0 || from >= size) {
                    return [];
                } else {
                    value len = to < size
                            then to - from + 1
                            else size - from;
                    return Measure(first.neighbour(from), len);
                }
            } else {
                if (from < 0 || to >= size) {
                    return [];
                } else {
                    value len = from < size
                            then from - to + 1
                            else size - to;
                    return Measure(first.neighbour(to), len).reversed;
                }
            }
        }

        shared actual
        Element[] spanFrom(Integer from) {
            if (from <= 0) {
                return this;
            } else if (from < size) {
                return Measure(first.neighbour(from), size - from);
            } else {
                return [];
            }
        }

        shared actual
        Element[] spanTo(Integer to) {
            if (to < 0) {
                return [];
            } else if (to < size - 1) {
                return Measure(first, to);
            } else {
                return this;
            }
        }

        shared actual void each(void step(Element element)) {
            variable value current = first;
            variable value count = 0;
            while (count++<size) {
                step(current++);
            }
        }
    }

    "Produces a [[Range]] of adjacent [[Enumerable]] values
     generated by a [[first]] element, and a strictly positive
     [[size]], or returns the [[empty sequence|empty]] if
     `size <= 0`. The range includes all values whose offset
     from `first` is non-negative and less than the `size`.

     More precisely, if `x` and `first` are of `Enumerable`
     type `X`, and `size` is an integer, then `x in first:size`
     if and only if `0 <= x.offset(first) < size`.

     The _measure operator_ `:` is an abbreviation for
     `measure()`:

         for (i in start:size) { ... }
         for (char in '0':10) { ... }

     The measure operator accepts the first index and size of
     the range:

         0:5     // [0, 1, 2, 3, 4]

     If the size is nonpositive, the range is empty:

         0:0     // []
         5:0     // []
         0:-5    // []"
    shared Range<Element>|[] measure<Element>
            (Element first, Integer size)
            given Element satisfies Enumerable<Element>
            => size <= 0 then [] else Measure(first, size);