3.18 The search agents we have discussed make use of a complete model of the world to construct a solution that they then execute. Modify the depth-first search algorithm with repeated state checking so that an agent can use it to explore an arbitrary vacuum world even without a model of the locations of walls and dirt. It should not get stuck even with loops or dead ends.

You may also wish to have your agent construct an environment description of the type used by the standard search algorithms.