An application next istrm moves to the next element in the stream.
FAILURE
If the transition function supplied when building the stream fails on
the current state.
EXAMPLE
- val istrm = mk_istream (fn x => x+1) 0 (concat "gsym" o int_to_string);
> val it = <istream> : (int, string) istream
- next istrm;
> val it = <istream> : (int, string) istream
COMMENTS
Perhaps the type of next should be ('a,'b) istream -> unit.