code: func ConvertToTemplateExpression(templateData io.Reader,
  	additionalUserTemplateProperties map[string]interface{}) (*gocf.StringExpr, error) {
  	converter := &templateConverter{
  		templateReader:          templateData,
  		additionalTemplateProps: additionalUserTemplateProperties,
  	}
  	return converter.expandTemplate().parseData().results()
  }
docs: // ConvertToTemplateExpression transforms the templateData contents into
  // an Fn::Join- compatible representation for template serialization.
  // The templateData contents may include both golang text/template properties
  // and single-line JSON Fn::Join supported serializations.