def tileset_info(filename):
    """

    Return the bounds of this tileset. The bounds should encompass the entire
    width of this dataset.

    So how do we know what those are if we don't know chromsizes? We can assume
    that the file is enormous (e.g. has a width of 4 trillion) and rely on the
    browser to pass in a set of chromsizes
    """
    pass
