qsort nil nil qsort cons x xs append qsort filterlow last cons x xs cons x xs cons last cons x xs qsort filterhigh last cons x xs cons x xs filterlow n nil nil filterlow n cons x xs if1 ge n x n x xs if1 true n x xs filterlow n xs if1 false n x xs cons x filterlow n xs filterhigh n nil nil filterhigh n cons x xs if2 ge x n n x xs if2 true n x xs filterhigh n xs if2 false n x xs cons x filterhigh n xs ge x 0 true ge 0 s x false ge s x s y ge x y append nil ys ys append cons x xs ys cons x append xs ys last nil 0 last cons x nil x last cons x cons y xs last cons y xs 2.2 qsort cons x xs qsort filterlow last cons x xs cons x xs qsort cons x xs qsort filterhigh last cons x xs cons x xs if1 true n x xs filterlow n xs if1 false n x xs filterlow n xs if2 true n x xs filterhigh n xs if2 false n x xs filterhigh n xs filterhigh n cons x xs if2 ge x n n x xs qsort cons x xs filterhigh last cons x xs cons x xs ge s x s y ge x y qsort cons x xs last cons x xs filterlow n cons x xs if1 ge n x n x xs last cons x cons y xs last cons y xs append cons x xs ys append xs ys qsort cons x xs filterlow last cons x xs cons x xs qsort cons x xs append qsort filterlow last cons x xs cons x xs cons last cons x xs qsort filterhigh last cons x xs cons x xs qsort cons x xs last cons x xs filterhigh n cons x xs ge x n qsort cons x xs last cons x xs filterlow n cons x xs ge n x true append cons x xs ys append xs ys true s 1 0 append 2 1 0 if1 4 0 false 0 0 ge 2 0 filterlow 2 0 qsort 1 0 true 0 0 last 1 0 if1 4 0 if2 4 0 qsort 1 0 append 2 0 0 0 0 ge 2 0 last 1 0 nil 0 0 filterlow 2 0 cons 2 2 1 filterhigh 2 0 ys 0 0 filterhigh 2 0 if2 4 0 qsort cons x xs qsort filterhigh last cons x xs cons x xs qsort cons x xs qsort filterlow last cons x xs cons x xs true Failed! NaTT certifiable-1.6