Reading Joe's answer, I now understand your question. I'm afraid it had me completely baffled before, and the reason I was confused was the same as the reason you were confused - by talking about "tags" you had me thinking at the level of raw unparsed XML, whereas actually what you want to emit is an element node in the result tree. It's really important to get this straight. Incidentally, the construct <FOO>...</FOO> uses two tags to represent one element - a start tag and an end tag.
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference