Problem:
 U31(tt()) -> N
 U42(tt()) -> s(plus(N,M))
 U11(tt()) -> U12(isNat())
 U12(tt()) -> tt()
 U21(tt()) -> tt()
 U41(tt()) -> U42(isNat())
 isNat() -> tt()
 isNat() -> U11(isNat())
 isNat() -> U21(isNat())
 plus(N,0()) -> U31(isNat())
 plus(N,s(M)) -> U41(isNat())

Proof:
 Fresh Variable Processor: loop length: 1
                           terms:
                            U31(tt())
                           context: []
                           substitution:
                            N -> U31(tt())
  Qed