reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th16:
  for x, y, z being Element of L holds (x | (y | z)) | (x | z) = x
proof
  let x, y, z be Element of L;
  (z | z) | (y | z) = z by Th11;
  hence thesis by Th15;
end;
