code: func (f Index) CountFrom(start Item) (count int, err error) {
  	startKey, err := f.encodeKeyFunc(start)
  	if err != nil {
  		return 0, err
  	}
  	it := f.db.NewIterator()
  	defer it.Release()
  
  	for ok := it.Seek(startKey); ok; ok = it.Next() {
  		key := it.Key()
  		if key[0] != f.prefix[0] {
  			break
  		}
  		count++
  	}
  	return count, it.Error()
  }
docs: // CountFrom returns the number of items in index keys
  // starting from the key encoded from the provided Item.