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 Th120:
  for p,y holds (y | p) | ((y | y) | p) = (p | p) | (y | p)
proof
  let p,y;
  p | (y | p) = (y | y) | p by Th119;
  hence thesis by Th119;
end;
