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 Th81:
  for x,y holds y | (x | x) = (x | x) | y
proof
  now
    let p,x,y;
    (((y | (x | x)) | (y | (x | x))) | (p | (p | p))) = y | (x | x) by Th71;
    hence y | (x | x) = (x | x) | y by Th80;
  end;
  hence thesis;
end;
