U11 tt s length L and tt X zeros cons 0 isNat tt isNat isNatList isNat isNat isNatIList isNatList isNatIList tt isNatIList and isNat isNatList tt isNatList and isNat length nil 0 length cons N U11 and isNatList 2.2 NaTT certifiable-1.6