We keep the wall between us as we go. To each the boulders that have fallen to each. And some are loaves and some so nearly balls We have to use a spell to make them balance: "Stay where you are until our backs are turned!" We wear our fingers rough with handling them. Oh, just another kind of out-door game, One on a side. It comes to little more: There where it is we do not need the wall: He is all pine and I am apple orchard. What feature of this excerpt makes it blank verse?