Next we introduce negative numbers via the function symbol p (for predecessor or 1, just as s stands for successor or +1). Suppose p(X) were a numeral whenever X is a numeral. Modify the predicate add2 to add3 such that for instance, ?- add3(p(s(0)), s(s(0)), Z). Z = s(s(0)) ?- add3(0, s(p(0)), Z). Z = = 0 ? add3 (p(0)+s (s(0)),s(s(0)),Z). Z = s(s(s(0))) ?- add3(p(0), p(0)+s(p(0)), Z). Z = p(p(0)) so that Z takes the form sn (0) or pn (0) for some n 0.