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 2.2 NaTT certifiable-1.6