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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 get_children(self, usage_id_filter=None):
 """ Return instantiated XBlocks for each of this blocks ``children``. """

        if not self.has_children:
            return []

        return [
            self.get_child(usage_id)
            for usage_id in self.children
            if usage_id_filter is None or usage_id_filter(usage_id)
        ]