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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 _combined_services(cls):
 # pylint: disable=no-self-argument """ A dictionary that collects all _services_requested by all ancestors of this XBlock class. """

        # The class declares what services it desires. To deal with subclasses,
        # especially mixins, properly, we have to walk up the inheritance
        # hierarchy, and combine all the declared services into one dictionary.
        combined = {}
        for parent in reversed(cls.mro()):
            combined.update(getattr(parent, "_services_requested", {}))
        return combined