reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th28:
  for x, y, z being Element of L holds (x | (y | (x | z))) | y = y | (x | z)
proof
  let x, y, z be Element of L;
  set Y = y | (x | z);
  Y | (x | y) = y by Th25;
  hence thesis by Th24;
end;
