def _get_s_exon_paths(s_exon, path_list, s_exon2path):
    """
    Return the set of path with a given s_exon.

    It uses s_exon2path to memoize the result.
    """
    if s_exon not in s_exon2path:
        s_exon2path[s_exon] = set(
            filter(lambda x: '/' + s_exon + '/' in x, path_list))

    return s_exon2path[s_exon]