reserve L for satisfying_Sh_1 non empty ShefferStr;
reserve L for satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3
  non empty ShefferStr;
reserve v,q,p,w,z,y,x for Element of L;

theorem Th128:
  for z,y holds (y | ((y | y) | z)) | (y | ((y | y) | z)) = y
proof
  now
    let z,y,x;
    (((x | x) | x) | y) | ((z | z) | y) = y by Th127;
    hence (y | ((y | y) | z)) | (y | ((y | y) | z)) = y by Th114;
  end;
  hence thesis;
end;
