code: @SuppressWarnings("unchecked")
  	@Override
  	public IndexedSet<T> convert(Collection<?> c) {
  		if (c == null)
  			return empty();
  
  		// useless to convert...
  		if (hasSameIndices(c))
  			return (IndexedSet<T>) c;
  		
  		// NOTE: cannot call super.convert(c) because of loop
  		IndexedSet<T> res = empty();
  		for (T t : (Collection<T>) c) 
  			res.add(t);
  		return res;
  	}
docs: {@inheritDoc}