code: func (b *bundle) executeUserTemplate(templateName string, input *EmailTemplateInput) (subject, body string, err error) {
  	if b.err != nil {
  		err = b.err
  		return
  	}
  
  	var buf bytes.Buffer
  	if err = b.subjects.ExecuteTemplate(&buf, templateName, input); err != nil {
  		return
  	}
  	subject = buf.String()
  
  	buf.Reset()
  	if err = b.bodies.ExecuteTemplate(&buf, templateName, input); err != nil {
  		return
  	}
  	body = buf.String()
  	return
  }
docs: // executeUserTemplate executed a user-defined template.
  // If b.err is not nil, returns it right away.