code: func (it *Iterator) BufKey(b []byte) []byte {
  	k := it.RawKey()
  	if k == nil {
  		return nil
  	}
  	if b == nil {
  		b = []byte{}
  	}
  
  	b = b[0:0]
  	return append(b, k...)
  }
docs: // Copy key to b, if b len is small or nil, returns a new one.