reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th66:
  for x, y, z being Element of L holds ((x | x) | y) | ((z | z) |
  y) = (y | (x | z)) | (y | (x | z))
proof
  let x, y, z be Element of L;
  (y | (z | z)) | (y | (x | x)) = ((x | x) | y) | ((z | z) | y) by Th40;
  hence thesis by Th65;
end;
