code: func (c *PCPCounterVector) Inc(inc int64, instance string) error {
  	c.mutex.Lock()
  	defer c.mutex.Unlock()
  
  	if inc < 0 {
  		return errors.New("increment cannot be negative")
  	}
  
  	if inc == 0 {
  		return nil
  	}
  
  	v, err := c.valInstance(instance)
  	if err != nil {
  		return err
  	}
  
  	return c.setInstance(v.(int64)+inc, instance)
  }
docs: // Inc increments the value of a particular instance of PCPCounterVector.