2ndspos s N cons X 2ndspos s N cons2 Z 2ndspos s N cons2 cons Y rcons posrecip Y 2ndsneg N Z 2ndsneg s N cons X 2ndsneg s N cons2 Z 2ndsneg s N cons2 cons Y rcons negrecip Y 2ndspos N Z from X cons X 2ndspos 0 Z rnil 2ndsneg 0 Z rnil pi X 2ndspos X from 0 plus 0 Y Y plus s X Y s plus X Y times 0 Y 0 times s X Y plus Y times X Y square X times X X 2.2 NaTT certifiable-1.6