code: def arrow(self, JOIN, _id):
          
          r = JOIN
          for node in self.lattice.get_elements(JOIN):
              if node.left_hand_side == _id:
                  r = r ^ self.lattice.el2bv[node]
          return r
docs: Removes all previous assignments from JOIN that have the same left hand side.
          This represents the arrow id definition from Schwartzbach.