code: func substitute(c context.Context, s string, subs interface{}) (string, error) {
  	t, err := template.New("tmpl").Parse(s)
  	if err != nil {
  		return "", err
  	}
  	buf := bytes.Buffer{}
  	if err = t.Execute(&buf, subs); err != nil {
  		return "", nil
  	}
  	return buf.String(), nil
  }
docs: // substitute performs substitutions in a template string.