user_error, but only prints if Topic is activated through
debug/1. Args is a meta-argument to deal with goal for the
@-command. Output is first handed to the hook
prolog:debug_print_hook/3. If this fails, Format+Args is
translated to text using the message-translation (see
print_message/2) for the term debug(Format, Args) and then
printed to every matching destination (controlled by debug/1)
using print_message_lines/3.
The message is preceded by '% ' and terminated with a newline.