returns the implications {v:Us} based on B
        This is L=∪ min L_i in [1]_

def v_Us_B(self):
        """
        returns the implications {v:Us} based on B
        This is L=∪ min L_i in [1]_
        """
        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)