reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th64:
  for x, y, z being Element of L holds (x | (y | y)) | (x | (z | (
  x | (y | y)))) = (x | (z | y)) | (x | (z |y))
proof
  let x, y, z be Element of L;
  x | (y | y) = (y | y) | x by Th37;
  hence thesis by Th63;
end;
