code: def v_Us_B(self):
          
          Bg = reduce(lambda x,y:x&y,(B(g,self.width-1) for g in self))
          gw = self.width
          return v_Us_dict(Bg, gw)
docs: returns the implications {v:Us} based on B
          This is L=∪ min L_i in [1]_