def dominator_set_intersect(b1, b2, doms)
      finger1, finger2 = b1, b2
      while finger1.post_order_number != finger2.post_order_number
        finger1 = doms[finger1] while finger1.post_order_number < finger2.post_order_number
        finger2 = doms[finger2] while finger2.post_order_number < finger1.post_order_number
      end
      finger1
    end
def dominator_set_intersect(b1, b2, doms)
      finger1, finger2 = b1, b2
      while finger1.post_order_number != finger2.post_order_number
        finger1 = doms[finger1] while finger1.post_order_number < finger2.post_order_number
        finger2 = doms[finger2] while finger2.post_order_number < finger1.post_order_number
      end
      finger1
    end
performs a set intersection of the dominator tree.