add a heading w/ #+print_bibliography to parse tree on export

[Matt Price <moptop99@gmail.com>]

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Goal:
conditionally add a headline with content to parse tree using either
org-export-before-parsing-hook or org-export-filter-parse-tree-functions

Explanation:
I use org-re-reveal to make lecture slides from subtrees. Often I forget to
add a heading with #+print_bibliography: and as a result org-cite does not
generate the bibliography, and my citations are not wrapped in links. I'd
like to search *either* the parse tree  *or* the temporary export buffer
for the #+print_bibliography keyword, and if it's not found, add a headline
to the end of the subtree/parse-tree with the appropriate information.

I can easily generate the proper lisp object with the org-ml package:

(org-ml-build-headline :title '("Sources")
                       (org-ml-build-section
                        (org-ml-build-property-drawer
                         (org-ml-build-node-property "CUSTOM_ID"
"bibliography"))
                        (org-ml-build-keyword "print_bibliography" "" )
                        ))

And if I want I can turn that into a string by wrapping it in
(org-ml-to-trimmed-string ...)

However, I'm not sure how best to add it to the exported data.  I believe I
can add a function to either org-export-before-parsing-hook or
org-export-filter-parse-tree-functions, but beyond that I'm a bit lost.
It's not immediately obvious how to find the last headline and append
another headline after that headline's section. Looking at the nesting
confuses me somewhat and I'm afraid I'll break the data structure.

Has anyone done something like this before and do you have ideas?

Thank you!

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