Directive, to create infix, prefix, postfix predicates.
For example: :- op(600, xfx, myInfixPredicate).
First argument is the precedence. Has to be an integer between 0 and 1200.
Second argument is the type. Has to be one of: xf yf xfx xfy yfx fy fx
Third argument is the predicate that is defined in this directive.
Then, a myInfixPredicate b. is equivalent to myInfixPredicate(a, b).
Built-in predicates cannot be modified by user.