sum cons s n x cons m y sum cons n x cons s m y sum cons 0 x y sum x y sum nil y y empty nil true empty cons n x false tail nil nil tail cons n x x head cons n x n weight x if empty x empty tail x x if true b x weight_undefined_error if false b x if2 b x if2 true x head x if2 false x weight sum x cons 0 tail tail x 2.2 if2 false x weight sum x cons 0 tail tail x weight x if empty x empty tail x x weight x tail x if2 true x head x sum cons s n x cons m y sum cons n x cons s m y weight x empty x if false b x if2 b x if2 false x tail x if2 false x sum x cons 0 tail tail x sum cons 0 x y sum x y weight x empty tail x if2 false x tail tail x true weight x if empty x empty tail x x if false b x if2 b x if2 false x weight sum x cons 0 tail tail x true Failed! NaTT certifiable-1.6