reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th60:
  for x, y, z being Element of L holds x | (((y | (z | x)) | (y |
  (z | x))) | (z | z)) = x | (y | (z | x))
proof
  let x, y, z be Element of L;
  set Y = y | (z | x);
  z | (x | (x | Y)) = z | z by Th58;
  hence thesis by Th49;
end;
