code: func reallocBytes(b []byte, n int) []byte {
  	newSize := len(b) + n
  	if cap(b) < newSize {
  		bs := make([]byte, len(b), newSize)
  		copy(bs, b)
  		return bs
  	}
  
  	// slice b has capability to store n bytes
  	return b
  }
docs: // reallocBytes is like realloc.