reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th27:
  for x, y, z being Element of L holds x | (y | (x | (y | z))) = x | (y | z)
proof
  let x, y, z be Element of L;
  set Y = y | x;
  (x | (y | z)) | Y = x by Th25;
  hence thesis by Th25;
end;
