Return a bed file from the pybedtools examples directory.  Use
    :func:`list_example_files` to see a list of files that are included.

def example_filename(fn):
    """
    Return a bed file from the pybedtools examples directory.  Use
    :func:`list_example_files` to see a list of files that are included.
    """
    fn = os.path.join(data_dir(), fn)
    if not os.path.exists(fn):
        raise ValueError("%s does not exist" % fn)
    return fn