reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th61:
  for x, y, z being Element of L holds x | (y | (z | z)) = x | (y | (z | x))
proof
  let x, y, z be Element of L;
  (y | (z | x)) | (y | (z | x)) | (z | z) = y | (z | z) by Th48;
  hence thesis by Th60;
end;
