For debug/1, Topic can be a term Topic > Out, where Out is either a stream or stream-alias or a filename (an atom). This redirects debug information on this topic to the given output. On Linux systems redirection can be used to make the message appear, even if the user_error stream is redefined using

?- debug(Topic > '/proc/self/fd/2').

A platform independent way to get debug messages in the current console (for example, a swipl-win window, or login using ssh to Prolog running an SSH server from the libssh pack) is to use:

?- stream_property(S, alias(user_error)),
   debug(Topic > S).

Do not forget to disable the debugging using nodebug/1 before quitting the console if Prolog must remain running.