conditionally add heading to tree on export

[Matt Price <moptop99@gmail.com>]

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Hi everyone,

Now that org-cite is working (!!!), I am incrementally updating all my
lecture slides to use proper citations.  This is mostly quite simple, but I
find that in the rush before class I am often forgetting to add a new slide
with *both* the

#+print_bibliography:

line that will generate a bibliography, *and* a :CUSTOM_ID: property set to
~bibliography~, which is required for working links around citation
objects.

I'd like to check to see whether a slide with :CUSTOM_ID of ~bibliography~
exists, and if not, insert a heading at the end of the exported text
(either the file or a subtree), something like

** Sources
:PROPERTIES:
:CUSTOM_ID: bibliography
:END:
#+print_bibliography:


What is the appropriate way for me to do this as an end user who doesn't
want to write his own derived backend? Is the appropriate process to write
a function that inspects and modifies the parse tree, and then add that
function to ~org-export-filter-body-functions~?

In fact, I wonder if I want to do this for all HTML export that contains
citation objects. What do other people think about that idea?

Thanks as always,
Matt

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