These all succeed:
is_stream(user_input). is_stream(user_output). is_stream(user_error). is_stream(current_input). is_stream(current_output).
As does this:
?- open("/etc/motd",read,Stream),
is_stream(Stream),
atomic(Stream),
blob(Stream,T),
close(Stream),
blob(Stream,TT).
Stream = <stream>(0x29db6b0),
T = TT, TT = stream.