code: func (l *blocksLRU) add(b block) {
  	if _, ok := l.blocks[b.offset]; ok {
  		panic(fmt.Sprintf("block with offset %d is already in the LRU", b.offset))
  	}
  	l.prepareForAdd()
  	l.blocks[b.offset] = l.ll.PushFront(b)
  }
docs: // add adds a block to the cache (perhaps evicting the oldest block).
  //
  // The caller must verify there's no such block in the cache already using
  // get(). Panics if it wasn't done.