Problem:
 U11(tt()) -> N
 U21(tt()) -> s(plus(N,M))
 and(tt()) -> X
 isNat() -> tt()
 isNat() -> and(isNat())
 isNat() -> isNat()
 plus(N,0()) -> U11(isNat())
 plus(N,s(M)) -> U21(and(isNat()))

Proof:
 Fresh Variable Processor: loop length: 1
                           terms:
                            U11(tt())
                           context: []
                           substitution:
                            N -> U11(tt())
  Qed