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 Th91:
  for p,q holds p = ((((q | q) | (q | q)) | p) | ((q | q) | p))
proof
  let p,q;
  (p | p) | (p | ((q | q) | q)) = p by Th77;
  hence thesis by Th90;
end;
