XSB support for recursion over negation

There are two things that need to be done in order to support recursion over negation:

1: make sure all potential calls that can cause recursion over negation are ground (i.e. instantiated)

2: For all top-level queries (the ones that IDP asks XSB), include a detection that the answer tuples can be undefined. If one such answer tuple exists, there is no (two-valued) model of the definition and hence the definition is non-total. In this case, return "nil"