U41 tt N U52 tt s plus N M U72 tt plus x N M N U11 tt U12 isNat U12 tt tt U21 tt tt U31 tt U32 isNat U32 tt tt U51 tt U52 isNat U61 tt 0 U71 tt U72 isNat isNat tt isNat U11 isNat isNat U21 isNat isNat U31 isNat plus N 0 U41 isNat plus N s M U51 isNat x N 0 U61 isNat x N s M U71 isNat 2.2 NaTT certifiable-1.6