private IEnumerable<Length> SuggestConvertions (Length length)
    => new List<Length>
       {
           length.km(),
           length.hm(),
           length.dam(),
           length.m(),
           length.dm(),
           length.cm(),
           length.mm(),
           length.yd(),
           length.ft(),
           length.inch(),
       }.Where(l => l.OriginalUnit != length.OriginalUnit);