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.