reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th42:
  for x, y being Element of L holds x | (y | x) = x | (y | y)
proof
  let x, y be Element of L;
  set Y = x | (y | y);
  Y | (x | y) = x by Th16;
  hence thesis by Th38;
end;
