xml. How to describe an XSL that will calculate the factorial of a number?

Question

XSL & XSLT

Good day.
Help to implement factorial calculation in xml, please.
The xml document looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<factorial-of>5</factorial-of>

Output: just a number, in this example 120

Answer 1

Recursion???...