Equivalent for (tree-before?)

[Chris Maden <crism@xxxxxxxxxxx>]

I need to check which footnotes come before a page break:

(let ((fn-list (select-elements (descendants (current-node))
                                (norm "footnote")))
      (last-break-pi (node-list-last (find-break-pis))))
  (process-node-list (node-list-filter (lambda (cand)
                                         (tree-before? last-break-pi
                                                       cand))
                                       fn-list)))

I've implemented the reference functions from ISO/IEC 10179, and it's
*really* slow (presumably because of (sort-in-tree-order)).

Is there a faster way to do this?  In this case, I can use a variant
of (sort-in-tree-order) that only gets the (subtree) for the current
context (I only need to do this inside DocBook <sidebar>s), but it
would be nice to know another way.  When only elements are involved, I
just use (all-element-number), but that doesn't work when one of the
nodes is not an element.

Thanks in advance,
Chris
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