reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th56:
  for x, y, z, u being Element of L holds x | (y | (z | (z | (u |
  (y | x))))) = x | (y | y)
proof
  let x, y, z, u be Element of L;
  (y | x) | (z | (z | (u | (y | x))) | (y | x)) = z | (z | (u | (y | x)))
  by Th55;
  hence thesis by Th41;
end;
