code: func DescribeIterator(it Iterator) Description {
  	sz, exact := it.Size()
  	d := Description{
  		UID:  it.UID(),
  		Name: it.String(),
  		Type: it.Type(),
  		Tags: it.Tagger().Tags(),
  		Size: sz, Exact: exact,
  	}
  	if sub := it.SubIterators(); len(sub) != 0 {
  		d.Iterators = make([]Description, 0, len(sub))
  		for _, sit := range sub {
  			d.Iterators = append(d.Iterators, DescribeIterator(sit))
  		}
  	}
  	return d
  }
docs: // DescribeIterator returns a description of the iterator tree.