code: func DefaultLetsEncryptCache() string {
  	for _, f := range letsEncryptCacheFuncs {
  		if v := f(); v != "" {
  			return v
  		}
  	}
  	dir, err := PerkeepConfigDir()
  	if err != nil {
  		log.Fatalf("Could not compute DefaultLetsEncryptCache: %v", err)
  	}
  	return filepath.Join(dir, "letsencrypt.cache")
  }
docs: // DefaultLetsEncryptCache returns the path to the default Let's Encrypt cache
  // directory (or file, depending on the ACME implementation).