code: func (c *certificateCache) get(addr string) (ssh.Signer, bool) {
  	c.mu.Lock()
  	defer c.mu.Unlock()
  
  	certificate, ok := c.cache.Get(addr)
  	if !ok {
  		return nil, false
  	}
  
  	certificateSigner, ok := certificate.(ssh.Signer)
  	if !ok {
  		return nil, false
  	}
  
  	return certificateSigner, true
  }
docs: // get is goroutine safe and will return a ssh.Signer for a principal from
  // the cache.