le 0 y true le s x 0 false le s x s y le x y app nil y y app add n x y add n app x y low n nil nil low n add m x if_low le m n n add m x if_low true n add m x add m low n x if_low false n add m x low n x high n nil nil high n add m x if_high le m n n add m x if_high true n add m x high n x if_high false n add m x add m high n x head add n x n tail add n x x isempty nil true isempty add n x false quicksort x if_qs isempty x low head x tail x head x high head x tail x if_qs true x n y nil if_qs false x n y app quicksort x add n quicksort y 2.2 if_low false n add m x low n x if_qs false x n y app quicksort x add n quicksort y if_low true n add m x low n x low n add m x if_low le m n n add m x app add n x y app x y le s x s y le x y quicksort x if_qs isempty x low head x tail x head x high head x tail x quicksort x high head x tail x if_qs false x n y quicksort x low n add m x le m n if_high true n add m x high n x quicksort x tail x quicksort x head x quicksort x head x quicksort x head x quicksort x low head x tail x high n add m x if_high le m n n add m x if_high false n add m x high n x if_qs false x n y quicksort y quicksort x isempty x high n add m x le m n quicksort x tail x true quicksort x if_qs isempty x low head x tail x head x high head x tail x if_qs false x n y quicksort y if_qs false x n y quicksort x true Failed! NaTT certifiable-1.6