code: func (it *iterator) Key() []byte {
  	if len(it.keys) > 0 {
  		return []byte(it.keys[0])
  	}
  	return nil
  }
docs: // Key returns the key of the current key/value pair, or nil if done. The caller
  // should not modify the contents of the returned slice, and its contents may
  // change on the next call to Next.