"aunt" attributes

[John Fieber <jfieber@xxxxxxxxxxx>]

A question from a dsssl newbie.

Given a bit of a document:

ROOT -- CHAPT -- COMM -- P
              |       |- P
              |       +- P
              |
         [a2] |- COMM -- P
              |       |- P
              |       +- P
              |
         [a1] |- COMM -- P [1]
              |       |- P
              |       +- P
              |
              +- COMM -- P
                      |- P
                      +- P

In the construction rule for [1] (or any P), I want to know if
the value of a particular attribute of COMM [a1] is equal to the
same attribute of its previous sibling [a2]. 

How can I do this in jade (0.6)?  Digging in the DSSSL spec, it
looks a comparison between

  (attribute-string "myatt" (parent (current-node)))

and

  (attribute-string "myatt" (preced (parent (current-node))))

should do about what I'm after, but preced isn't implemented and
I'm not expert enough to make it work. 

Thanks,

- -john