code: @Override
    public ConciseSet convert(Collection<Integer> c)
    {
      ConciseSet res = empty();
      Collection<Integer> sorted;
      if (c != null) {
        if (c instanceof SortedSet<?> && ((SortedSet<?>) c).comparator() == null) {
          sorted = c;
        } else {
          sorted = new ArrayList<Integer>(c);
          Collections.sort((List<Integer>) sorted);
        }
        for (int i : sorted) {
          if (res.last != i) {
            res.add(i);
          }
        }
      }
      return res;
    }
docs: {@inheritDoc}