selection_properties.go
func (s *Selection) Attribute(attribute string) (string, error) {
	return s.hasProperty(element.Element.GetAttribute, attribute, "attribute")
}
func (s *Selection) Attribute(attribute string) (string, error) {
	return s.hasProperty(element.Element.GetAttribute, attribute, "attribute")
}
// Attribute returns an attribute value for exactly one element.