I just spent waaaaaay too long figuring this out, so I'll leave a note of it for future projects:

To enumerate the current node's position in the current nodeset, position() is great.  However when using nested nodesets, getting the parent's position relative to its nodeset is a bit more complex:

count(parent::*/preceding-sibling::*) + 1 - assuming all nodes in the parent nodeset are of the same type, otherwise:

count(parent::*/preceding-sibling::<type>) + 1 where <type> is the type of the parent node

A quick beakdown:

• parent::* - selects the parent of the current node
• parent::*/preceding-sibling::* - selects the nodes which precede the parent in its nodeset
• count(parent::*/preceding-sibling::*) - counts the nodes preceding the parent
• count(parent::*/preceding-sibling::*) + 1 - if there are n nodes before the parent, then the parent is # n+1