code: func (c *Client) Incr(vb uint16, key string,
  	amt, def uint64, exp int) (uint64, error) {
  
  	req := &gomemcached.MCRequest{
  		Opcode:  gomemcached.INCREMENT,
  		VBucket: vb,
  		Key:     []byte(key),
  		Extras:  make([]byte, 8+8+4),
  	}
  	binary.BigEndian.PutUint64(req.Extras[:8], amt)
  	binary.BigEndian.PutUint64(req.Extras[8:16], def)
  	binary.BigEndian.PutUint32(req.Extras[16:20], uint32(exp))
  
  	resp, err := c.Send(req)
  	if err != nil {
  		return 0, err
  	}
  
  	return binary.BigEndian.Uint64(resp.Body), nil
  }
docs: // Incr increments the value at the given key.