?-  struct("s/1");

plus(0, Y) :- Y;
plus(s(X), Y) :- s(plus(X, Y));

mult(0, Y) :- 0;
mult(s(X), Y) :- plus(Y, mult(X,Y));

fact(0) :- s(0);
fact(s(X)) :- mult(s(X), fact(X));

calc(0) :- 0;
calc(s(X)) :- 1+calc(X);