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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def is_solved(self):
 """ Check if the cross of Cube is solved. """

        return self.cross_goal(({f: self.cube[f] for f in "LUFDRB"}, 
            self.cube.select_type("edge") & self.cube.has_colour(self.cube["D"].colour)))