code: func SignerFromConfig(c cli.Config) (ocsp.Signer, error) {
  	//if this is called from serve then we need to use the specific responder key file
  	//fallback to key for backwards-compatibility
  	k := c.ResponderKeyFile
  	if k == "" {
  		k = c.KeyFile
  	}
  	return ocsp.NewSignerFromFile(c.CAFile, c.ResponderFile, k, time.Duration(c.Interval))
  }
docs: // SignerFromConfig creates a signer from a cli.Config as a helper for cli and serve