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.