Forgive me for the tangent, especially if it sounds ignorant, but as a
newbie to XSL I have a hard time understanding the benefit in not
being able to redefine a variable.  While I do not doubt the talent
and genius that went into the above examples and speculations, I can't
imagine a benefit that outweighs the time economics involved in coming
up with the above solutions, versus simply doing a $variable =
$variable - $x and being done with it in 10ish lines.