a C x1 x2 y C a x1 y a x2 C x1 x2 a Z y Z eqZList C x1 x2 C y1 y2 and eqZList x1 y1 eqZList x2 y2 eqZList C x1 x2 Z False eqZList Z C y1 y2 False eqZList Z Z True second C x1 x2 x2 first C x1 x2 x1 and False False False and True False False and False True False and True True True a 2 second 1 eqZList 2 Z 0 True 0 C 2 and 2 first 1 False 0 INNERMOST