reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th63:
  for x, y, z being Element of L holds (x | (y | y)) | (x | (z | (
  (y | y) | x))) = (x | (z | y)) | (x | (z | y))
proof
  let x, y, z be Element of L;
  (x | (z | ((y | y) | x))) = (x | (z | y)) by Th62;
  hence thesis by Th45;
end;
