% Here is my translation of NatNum into Prolog, to give you the
  idea...
%  natnum(0).
%  natnum(N) :- s(M,N),natnum(M).


?- natnum(4).
true .

?- natnum(-2).
false.

?- natnum(0).
true .

?- s(X,4).
X = 3 .

?- s(4,X).
X = 5.

?- plus(3,4,X).
X = 7 .

?- mult(12,8,OUT).
OUT = 96 .

?- exp(2,8,Res).
Res = 256 .

?- plus(-2,3,Out).
false.

?-