code: func (bg *Reader) cacheHasBlockFor(base int64) (exists bool, next int64) {
  	bg.mu.RLock()
  	defer bg.mu.RUnlock()
  	if bg.cache == nil {
  		return false, -1
  	}
  	return bg.cache.Peek(base)
  }
docs: // cacheHasBlockFor returns whether the Reader's cache has a block
  // for the given base offset. If the requested Block exists, the base
  // offset of the following Block is returned.