code: func (c *cache) OnEvicted(f func(string, interface{})) {
  	c.mu.Lock()
  	c.onEvicted = f
  	c.mu.Unlock()
  }
docs: // Sets an (optional) function that is called with the key and value when an
  // item is evicted from the cache. (Including when it is deleted manually, but
  // not when it is overwritten.) Set to nil to disable.