PL_warning()
prints a standard Prolog warning message to the standard error (user_error)
stream. Please note that new code should consider using PL_raise_exception()
to raise a Prolog exception. See also section
4.10.
[WARNING: ’,
followed by the output from format, followed by a‘]’and
a newline. Then start the tracer. format and the arguments
are the same as for printf(2). Always returns FALSE.