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 Th73:
  for q,p,y,w holds (((w | (y | (y | y))) | p) | ((q | q) | p)) =
  ((p | (w | q)) | (p | (w | q)))
proof
  let q,p,y,w;
  (w | w) = (w | (y | (y | y))) by SHEFFER1:def 14;
  hence thesis by SHEFFER1:def 15;
end;
