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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 collection(self, collection_id):
 """Create a sub-collection underneath the current document. Args: collection_id (str):
 The sub-collection identifier (sometimes referred to as the "kind"). Returns: ~.firestore_v1beta1.collection.CollectionReference: The child collection. """

        child_path = self._path + (collection_id,)
        return self._client.collection(*child_path)