code: func (s *shard) Add(key uint32, el interface{}) {
  	l := s.Len()
  	if l+1 > s.size {
  		s.Evict()
  	}
  
  	s.Lock()
  	s.items[key] = el
  	s.Unlock()
  }
docs: // Add adds element indexed by key into the cache. Any existing element is overwritten