code: func loadX509KeyPair(certFile, keyFile string) (cert tls.Certificate, err error) {
  	certPEMBlock, err := wkfs.ReadFile(certFile)
  	if err != nil {
  		return
  	}
  	keyPEMBlock, err := wkfs.ReadFile(keyFile)
  	if err != nil {
  		return
  	}
  	return tls.X509KeyPair(certPEMBlock, keyPEMBlock)
  }
docs: // loadX509KeyPair is a copy of tls.LoadX509KeyPair but using wkfs.