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