def _filter_index_pages(docnames, base_dir):
    """Filter docnames to only yield paths of the form
    ``<base_dir>/<name>/index``

    Parameters
    ----------
    docnames : `list` of `str`
        List of document names (``env.found_docs``).
    base_dir : `str`
        Base directory of all sub-directories containing index pages.

    Yields
    ------
    docname : `str`
        Document name that meets the pattern.
    """
    for docname in docnames:
        parts = docname.split('/')
        if len(parts) == 3 and parts[0] == base_dir and parts[2] == 'index':
            yield docname
