code: func (c *PCPCounter) Inc(val int64) error {
  	c.mutex.Lock()
  	defer c.mutex.Unlock()
  
  	if val < 0 {
  		return errors.New("cannot decrement a counter")
  	}
  
  	if val == 0 {
  		return nil
  	}
  
  	v := c.val.(int64)
  	v += val
  	return c.set(v)
  }
docs: // Inc increases the stored counter's value by the passed increment.