def _add_digest(self, signature):
        if self.digest_history_size == 0:
            # no history, never add digests
            return

        self.digest_history.add(signature)
        if len(self.digest_history) > self.digest_history_size:
            # threshold reached, cull 10%
            self._cull_digest_history()
Called in the EnvironBuilder to add files from the data dict.
dataset/ETHPy150Open jupyter/jupyter_client/jupyter_client/session.py/Session._add_digest--crossover with--dataset/ETHPy150Open Eforcers/gae-flask-todo/lib/werkzeug/test.py/EnvironBuilder._add_file_from_data