reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th55:
  for x, y, z being Element of L holds x | ((y | (y | (z | x))) |
  x) = y | (y | (z | x))
proof
  let x, y, z be Element of L;
  y | (z | x) = (x | (y | (x | z))) | y by Th30;
  hence thesis by Th54;
end;
