"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();