operator
key of specified? attributes (of element-expression)? [numeric-expression]
The key of attribute operator returns the "true" name of the attribute. This is most useful when
        retrieving the real name of an attribute being referenced through an alias. The name is uppercased when the SGML
        declaration specifies
        NAMECASE GENERAL YES for the SGML document being processed.
      
When key of is applied to attributes, the item indexer is
        required.