code: func (c *LRU) Peek(base int64) (exist bool, next int64) {
  	c.mu.RLock()
  	defer c.mu.RUnlock()
  
  	n, exist := c.table[base]
  	if !exist {
  		return false, -1
  	}
  	next = n.b.NextBase()
  	return exist, next
  }
docs: // Peek returns a boolean indicating whether a Block exists in the Cache for
  // the given base offset and the expected offset for the subsequent Block in
  // the BGZF stream.