X represents the name and arity of the predicate. Y represents the full class name of an implementation of org.projog.core.ArithmeticOperator.
Examples
?- X is sum(1, 1).
Cannot find arithmetic operator: sum/2
?- pj_add_arithmetic_operator(sum/2, 'org.projog.core.math.builtin.Add').
yes
?- X is sum(1, 1).
X = 2
yes