Problem:
 U52(tt()) -> N
 U64(tt()) -> s(plus(N,M))
 U11(tt()) -> U12(isNatKind())
 U12(tt()) -> U13(isNatKind())
 U13(tt()) -> U14(isNatKind())
 U14(tt()) -> U15(isNat())
 U15(tt()) -> U16(isNat())
 U16(tt()) -> tt()
 U21(tt()) -> U22(isNatKind())
 U22(tt()) -> U23(isNat())
 U23(tt()) -> tt()
 U31(tt()) -> U32(isNatKind())
 U32(tt()) -> tt()
 U41(tt()) -> tt()
 U51(tt()) -> U52(isNatKind())
 U61(tt()) -> U62(isNatKind())
 U62(tt()) -> U63(isNat())
 U63(tt()) -> U64(isNatKind())
 isNat() -> tt()
 isNat() -> U11(isNatKind())
 isNat() -> U21(isNatKind())
 isNatKind() -> tt()
 isNatKind() -> U31(isNatKind())
 isNatKind() -> U41(isNatKind())
 plus(N,0()) -> U51(isNat())
 plus(N,s(M)) -> U61(isNat())

Proof:
 Fresh Variable Processor: loop length: 1
                           terms:
                            U52(tt())
                           context: []
                           substitution:
                            N -> U52(tt())
  Qed