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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 _firestore_api(self):
 """Lazy-loading getter GAPIC Firestore API. Returns: ~.gapic.firestore.v1beta1.firestore_client.FirestoreClient: The GAPIC client with the credentials of the current client. """

        if self._firestore_api_internal is None:
            self._firestore_api_internal = firestore_client.FirestoreClient(
                credentials=self._credentials
            )

        return self._firestore_api_internal