<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def solve(self):
 """ Solve the cross. """

        result = Formula(path_actions(a_star_search(
            ({f: self.cube[f] for f in "LUFDRB"}, 
             self.cube.select_type("edge") & self.cube.has_colour(self.cube["D"].colour)), 
            self.cross_successors, 
            self.cross_state_value, 
            self.cross_goal, 
            )))
        self.cube(result)
        return result