reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th2:
  for x, y, z being Element of L holds ((x | y) | (((y | ((z | y) |
  y)) | (x | y)) | (x | y))) | z = y | ((z | y) | y)
proof
  let x, y, z be Element of L;
  set X = x | y;
  set Y = y | ((z | y) | y);
  (X | ((Y | X) | X)) | (Y | (z | X)) = Y by Def1;
  hence thesis by Def1;
end;
