I would use an external variable and a set of functions (increase,
decrease, getValue) defined using the script element from the
http://www.exslt.org/functions namespace.

I think when you want to increment/decrement a variable, you may be
justified to escape to a procedural language and to increment/decrement
a variable simply. XSLT is not designed for this task and all other ways
are more or less workarounds, IMHO.

I see this is not a "pure XSLT" solution but I think it's appropriate
for this non-XSLT task.

Following this line of argument it may be better not to use XSLT at all.

You *are* aware that using extension functions to change variable values not
only ties you to a specific extension, but also to a particular
implementation/version (execution order, variable evaluation order...)?

--
Petr Cimprich
Ginger Alliance
www.gingerall.com