I considered doing a fallthough in the case-statement, but that doesn't work on bash leq 4 (notably, those versions shipped with MacOS by default).
I chose to prepend the value to the array to mirror the homebrew completion [instructions page](https://docs.brew.sh/Shell-Completion).
I also chose to leave off the `-d`irectory check -- zsh seems to tolerate invalid paths, so there isn't much harm in adding it anyway.
I'm flexible on any of these choices should someone feel strongly. However, I imagine this is the best combination given the trade-offs we have.
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