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 Th156:
  for w,z,x holds w | x = w | ((x | z) | (x | w))
proof
  now
    let y,w,z,x;
    (x | (y | (y | y))) | (x | (y | (y | y))) = x by Th136;
    hence w | x = w | ((x | z) | (x | w)) by Th155;
  end;
  hence thesis;
end;
