[O] merging per-file (or heading) TODO sequences

[Eric Abrahamsen]

I could have sworn I've done this successfully before, but...

I need a special sequence of TODO keywords for one file -- actually just
one headline in particular. I thought I remembered this used to be
possible, but it doesn't seem to be anymore.

Setting #+SEQ_TODO: FOO | BAR at the top of the file actually
overwrites the existing value of `org-todo-keywords' for that file. In
this case I'd like to either:

1. Create this sequence for a single subtree (it's okay if it clobbers
the global value of `org-todo-keywords', so long as it's *only* for this
subtree), or

2. Set the new sequence at the top of the file, but merge this sequence
with the global value of `org-todo-keywords'.

I was sure this was possible! What am I missing?

Thanks,
Eric