code: func (c *LRUExpireCache) Add(key interface{}, value interface{}, ttl time.Duration) {
  	c.lock.Lock()
  	defer c.lock.Unlock()
  	c.cache.Add(key, &cacheEntry{value, c.clock.Now().Add(ttl)})
  }
docs: // Add adds the value to the cache at key with the specified maximum duration.