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 Th75:
  for w,p,y,q holds (((w | w) | p) | ((q | (y | (y | y))) | p)) =
  ((p | (w | q)) | (p | (w | q)))
proof
  let w,p,y,q;
  q | q = q | (y | (y | y)) by SHEFFER1:def 14;
  hence thesis by SHEFFER1:def 15;
end;
