    "Given zero or more argument [[streams|iterables]], return a
     new sequence containing all elements of every given stream.
     The elements of the resulting stream are ordered first
     according to the stream in which they occur, and then
     according to where they occur in that stream. If there are
     no arguments, or if none of the argument streams contain
     any elements, return the empty sequence.

     For example, the expression

         concatenate(1..3, [0.0], {\"hello\", \"world\"})

     results in the sequence `[1, 2, 3, 0.0, \"hello\", \"world\"]`
     which has the type `[Integer|Float|String*]`."
    see (`function expand`,
         `function Iterable.chain`,
         `function Sequential.append`)
    tagged("Streams")
    shared Element[] concatenate<Element>(
            "The streams to concatenate."
            {Element*}* iterables)
            => [ for (it in iterables) for (val in it) val ];