Problem:
 fact(X) -> if(zero(X),s(0()),prod(X,fact(p(X))))
 add(0(),X) -> X
 add(s(X),Y) -> s(add(X,Y))
 prod(0(),X) -> 0()
 prod(s(X),Y) -> add(Y,prod(X,Y))
 if(true(),X,Y) -> X
 if(false(),X,Y) -> Y
 zero(0()) -> true()
 zero(s(X)) -> false()
 p(s(X)) -> X

Proof:
 Containment Processor:
  loop length: 1
  terms:
   fact(X)
  context: if(zero(X),s(0()),prod(X,[]))
  substitution:
   X -> p(X)
  Qed