Associates an alias with a parsed XML document that is held in-memory after it is first referred to. In subsequent calls to USXSL.Apply and USXSL.Apply2File, you can refer to the XML document by using the alias.
The point of using this method is that, if you need the same XSL document more than once, referring to its parsed version by alias may improve performance.
Returns the alias as a string. If the passed XSL document is invalid, an error is returned.
If xml-document is empty, the alias-document mapping is removed from the list.