These functions had very similar bugs: they didn't check if their chosen
"close token" was already present in the string they're escaping.
I went ahead and did the work implied by the TODOs: search for a "close
token" that is *not* in the original string. Pretty simple concept, but
it turned into an annoying amount of code. I couldn't find anything in
upstream nixpkgs lib, or some clever insight about lua/vimscript that
makes this work unecessary, but I'll be thrilled (and a little bummed
about a wasted afternoon) to learn about something.
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