code: func (c *cache) IncrementInt(k string, n int) (int, error) {
  	c.mu.Lock()
  	v, found := c.items[k]
  	if !found || v.Expired() {
  		c.mu.Unlock()
  		return 0, fmt.Errorf("Item %s not found", k)
  	}
  	rv, ok := v.Object.(int)
  	if !ok {
  		c.mu.Unlock()
  		return 0, fmt.Errorf("The value for %s is not an int", k)
  	}
  	nv := rv + n
  	v.Object = nv
  	c.items[k] = v
  	c.mu.Unlock()
  	return nv, nil
  }
docs: // Increment an item of type int by n. Returns an error if the item's value is
  // not an int, or if it was not found. If there is no error, the incremented
  // value is returned.