X represents the name and arity of the expression. Y represents the full class name of an implementation of org.projog.core.predicate.builtin.clp.ExpressionFactory.
Examples
?- 7 #= sum(X, 4).
Cannot find CLP expression: sum/2
?- pj_add_clp_expression(sum/2, 'org.projog.core.predicate.builtin.clp.CommonExpression/add').
yes
?- 7 #= sum(X, 4).
X = 3
yes