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 Th72:
  for y,w holds (w | w) | (w | (y | (y | y))) = w
proof
  let y,w;
  w | w = w | (y | (y | y)) by SHEFFER1:def 14;
  hence thesis by SHEFFER1:def 13;
end;
