$4$ Like DFS$,$ online DFS is incomplete for reversible state spaces with infinite paths. For mple, suppose that states are points on the infinite two$-$dimensional grid and actions are vectors $(1,0),(0,1),(-1,0),(0,-1),$ tried in that order. Show that online DFS starting $0,0)$ will not reach $(1,-1).$ Suppose the agent can observe, in addition to its current $,$ all successor states and the actions that would lead to them. Write an algorithm that omplete even for bidirected state spaces with infinite paths. What states does it visit in hing $(1,-1)?$