reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th45:
  for x, y, z being Element of L holds (x | (y | y)) | (x | (z | y
  )) = (x | (z | y)) | (x | (z | y))
proof
  let x, y, z be Element of L;
  set Y = y;
  set X = x;
  (Y | X) | X = X | (Y | Y) by Th43;
  hence thesis by Th39;
end;
