code: func (c *RawBinaryClient) Append(key string, value []byte) MutateResponse {
  	item := &Item{
  		Key:   key,
  		Value: value,
  	}
  
  	c.mutex.Lock()
  	defer c.mutex.Unlock()
  
  	if resp := c.sendMutateRequest(opAppend, item, false); resp != nil {
  		return resp
  	}
  
  	return c.receiveMutateResponse(opAppend, item.Key)
  }
docs: // See Client interface for documentation.