reserve a,b,c,d,e,f,g,h for set;

theorem
  NOR8(a,b,c,d,e,f,g,h) is non empty iff not (a is non empty or b is non
  empty or c is non empty or d is non empty or e is non empty or f is non empty
  or g is non empty or h is non empty) by Def33;
