code: def solve(self):
          
          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
docs: Solve the cross.