    "Produces the elements of an [[Iterable]] object."
    see (`interface Iterable`)
    by ("Gavin")
    shared interface Iterator<out Element> {
        "The next element, or [[finished]] if there are no more
         elements to be iterated.

         Repeated invocations of `next()` for a given iterator
         must eventually produce any given element of the stream
         to which the iterator belongs. A given iterator must
         not produce the same element of the stream more often
         than the element occurs in the stream.

         If an invocation of `next()` for a given iterator
         produces the value `finished`, then every future
         invocation of `next()` for that iterator must also
         produce the value `finished`.

         An iterator for a nonfinite stream may never produce
         the value `finished`."
        shared formal Element|Finished next();
    }