Internal predicates

• prec/2 is used as the internal predicate indicating preference; it is thus the internal counterpart of </2.
• Predicate symbols starting with neg_ are reserved for (internally) expressing classical negation.
• Predicate symbols ap/1, bl/1, ok/1 and oko/2 are reserved for internal purposes.
• When dealing with set-ordered logic programs, as described in Section , we reserve moreover: nap/1, napo/2, nhead/1, ko/1, in/2 and sname/1.