def common_parent(coords, parent_zoom):
    """
    Return the common parent for coords

    Also check that all coords do indeed share the same parent coordinate.
    """
    parent = None
    for coord in coords:
        assert parent_zoom <= coord.zoom
        coord_parent = coord.zoomTo(parent_zoom).container()
        if parent is None:
            parent = coord_parent
        else:
            assert parent == coord_parent
    assert parent is not None, 'No coords?'
    return parent
Return the common parent for coords

    Also check that all coords do indeed share the same parent coordinate.