def clean_route_short_names(feed: "Feed") -> "Feed":
    """
    In ``feed.routes``, assign 'n/a' to missing route short names and
    strip whitespace from route short names.
    Then disambiguate each route short name that is duplicated by
    appending '-' and its route ID.
    Return the resulting "Feed".
    """
    feed = feed.copy()
    r = feed.routes
    if r is None:
        return feed

    # Fill NaNs and strip whitespace
    r["route_short_name"] = r["route_short_name"].fillna("n/a").str.strip()

    # Disambiguate
    def disambiguate(row):
        rsn, rid = row
        return rsn + "-" + rid

    r["dup"] = r["route_short_name"].duplicated(keep=False)
    r.loc[r["dup"], "route_short_name"] = r.loc[
        r["dup"], ["route_short_name", "route_id"]
    ].apply(disambiguate, axis=1)
    del r["dup"]

    feed.routes = r
    return feed
In ``feed.routes``, assign 'n/a' to missing route short names and
    strip whitespace from route short names.
    Then disambiguate each route short name that is duplicated by
    appending '-' and its route ID.
    Return the resulting "Feed".