Fix supertightness check

src/problem.rs

@@ -68,8 +68,8 @@ impl Problem
 
 			// If a backward proof is necessary, the program needs to be supertight, that is, no
 			// private predicates may transitively depend on themselves
-			if proof_direction.requires_backward_proof() && !predicate_declaration.is_public()
-				&& predicate_declaration.is_self_referential()
+			if proof_direction.requires_backward_proof()
+				&& predicate_declaration.has_private_dependency_cycle()
 			{
 				return Err(crate::Error::new_private_predicate_cycle(
 					std::rc::Rc::clone(&predicate_declaration)));