f true xs f eq s length xs length cons a xs append cons b nil xs length nil 0 length cons x xs s length xs eq 0 0 true eq s x 0 false eq 0 s y false eq s x s y eq x y append xs ys appendAkk reverse xs ys appendAkk nil ys ys appendAkk cons x xs ys appendAkk xs cons x ys reverse nil nil reverse cons x xs append reverse xs cons x nil 2.2 reverse cons x xs reverse xs f true xs f eq s length xs length cons a xs append cons b nil xs f true xs length cons a xs append xs ys appendAkk reverse xs ys append xs ys reverse xs f true xs eq s length xs length cons a xs f true xs append cons b nil xs length cons x xs length xs eq s x s y eq x y reverse cons x xs append reverse xs cons x nil f true xs length xs appendAkk cons x xs ys appendAkk xs cons x ys true f true xs f eq s length xs length cons a xs append cons b nil xs true Failed! NaTT certifiable-1.6