if true X Y X if false X Y Y lteq s M o false lteq o M true lteq s M s K lteq M K from M nil nil from M cons H Z if lteq M H cons H Z from M Z chain F nil nil chain F cons H Z cons F H chain F from F H Z incch X chain x N s x X X list Y list Z list M N K N H N F N N o N s N N true B false B nil list cons N list list if B list list list lteq N N B from N list list chain N N list list incch list list FULL .