(18,72)→ ¬Y;AND THUS, WE GET A CONTRADICTION AT 72

18: (11,18)→ ¬x; 17 : (18,17)→ ¬y; 72: (17,72)→ ¬z; 72: (18,72)→ ¬y;and thus, we get a contradiction at 72.[12, z],[19, x] :