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.