How to use the phylib.utils._as_list function in phylib

The new metadata value for the given clusters.
        add_to_stack : boolean
            Whether this metadata change should be recorded in the undo stack.

        Returns
        -------

        up : UpdateInfo instance

        """
        # Add the field if it doesn't exist.
        if field not in self._fields:
            self.add_field(field)
        assert field in self._fields

        clusters = _as_list(clusters)
        for cluster in clusters:
            if cluster not in self._data:
                self._data[cluster] = {}
            self._data[cluster][field] = value

        up = UpdateInfo(description='metadata_' + field,
                        metadata_changed=clusters,
                        metadata_value=value,
                        )
        undo_state = emit('request_undo_state', self, up)

        if add_to_stack:
            self._undo_stack.add((clusters, field, value, up, undo_state))
            emit('cluster', self, up)

        return up