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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 cross_successors(state, last_action=None):
 """ Successors function for solving the cross. """

        centres, edges = state
        acts = sum([
            [s, s.inverse(), s * 2] for s in
            map(Step, "RUFDRB".replace(last_action.face if last_action else "", "", 1))
            ], [])
        for step in acts:
            yield step, (centres, CrossSolver._rotate(edges, step))