    "Given a [[stream|iterables]] whose elements are also
     streams, return a new stream with all elements of every
     nested stream. If there are no nested streams, or if all of
     the nested streams are empty, return an empty stream.

     For example, the expression

         expand { 1..3, {5}, \"hi\" }

     results in the stream `{ 1, 2, 3, 5, 'h', 'i' }` which has
     the type `{Integer|Character*}`."
    see (`function Iterable.flatMap`,
        `function concatenate`,
        `function Iterable.chain`)
    tagged("Streams")
    shared Iterable<Element,OuterAbsent|InnerAbsent>
            expand<Element,OuterAbsent,InnerAbsent>
            (Iterable<Iterable<Element,InnerAbsent>,OuterAbsent> iterables)
            given OuterAbsent satisfies Null
            given InnerAbsent satisfies Null
            => { for (it in iterables) for (val in it) val };