preceeding template count

Hello

I am curently doing some work for the openoffice docbook filters, and i have a
problem (as you may have guessed ;-) Lets say i have a template matching
"a/b/c", and inside this template i want to get the number of "a" items before
the current one. I tried some mix of preceding-sibling, ancestor, count, and
../.. but i cannot seemd to get the number i want. Any idea ?

Thank you anyway, even if no one can help me !
Nicolas