def get_field_names_at_timestep(self, timestep):
        """
        Return the field names at a certain timestep.
        :param timestep: The timestep to return the field names at.
        :return: A list of all the field names at that timestep.
        """
        if timestep not in self._fields:
            return []
        return [field.name for field in self._fields[timestep]]

        Return the field names at a certain timestep.
        :param timestep: The timestep to return the field names at.
        :return: A list of all the field names at that timestep.
        
petastorm/ngram.py