reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th57:
  for x, y, z being Element of L holds x | (y | (y | (z | (x | y))
  )) = x | (y | (x | x))
proof
  let x, y, z be Element of L;
  y | (x | (y | (y | (z | (x | y))))) = y | (x | x) by Th56;
  hence thesis by Th27;
end;
