Operator.createOperator(arg, ops, mc);
assertEquals(1, ops.size());
assertTrue(ops.iterator().next().getMapping() instanceof RDFPredicateMapping);
// anything else --> NullFilter
arg = mc.valueOf(42);
ops.clear();
Operator.createOperator(arg, ops, mc);
assertEquals(1, ops.size());
assertTrue(ops.iterator().next().getMapping() instanceof NullStackMapping);