code: func (c *Client) Append(vb uint16, key string, data []byte) (*gomemcached.MCResponse, error) {
  	req := &gomemcached.MCRequest{
  		Opcode:  gomemcached.APPEND,
  		VBucket: vb,
  		Key:     []byte(key),
  		Cas:     0,
  		Opaque:  0,
  		Body:    data}
  
  	return c.Send(req)
  }
docs: // Append data to the value of a key.