U11 tt N U21 tt s plus N M U41 tt plus x N M N and tt X U31 tt 0 isNat tt isNat and isNat isNat isNat plus N 0 U11 isNat plus N s M U21 and isNat x N 0 U31 isNat x N s M U41 and isNat 2.2 NaTT certifiable-1.6