CurrentOutput
Implements ISO's current_output/1: unifies the argument with the $stream(...) term of the current output stream (the context's current output, or its standard output if none was SetOutput) if the argument is unbound; if it is already bound to a $stream(...) term, succeeds iff it already denotes that same channel.
Throws
if the argument names a well-formed stream term for which no channel is currently open.
(stream_or_alias) if it is bound to something other than a $stream(...) term.