is_odd(X) :- mod(X,2)=1. is_even(X) :- mod(X,2)=0.
?-is_odd(4) ?-is_odd
GNU Prolog 1.3.1
By Daniel Diaz
Copyright (C) 1999-2009 Daniel Diaz
uncaught exception: error(syntax_error('user_input:1 (char:3) . or operator expected after expression'),read_term/3)