src/main/java/com/conveyal/gtfs/error/ReferentialIntegrityError.java
@Override
    public int compareTo (GTFSError o) {
        int compare = super.compareTo(o);
        if (compare != 0) return compare;
        return this.badReference.compareTo((((ReferentialIntegrityError) o).badReference));
    }
@Override
    public int compareTo (GTFSError o) {
        int compare = super.compareTo(o);
        if (compare != 0) return compare;
        return this.badReference.compareTo((((ReferentialIntegrityError) o).badReference));
    }
must be comparable to put into mapdb