Whilst a thorough test suite is invaluable, I'm tempted to just hack on the code until they pass, instead of understanding the root issue.
miniblog.
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I find zoxide invaluable for jumping between frequently used directories: https://github.com/ajeetdsouza/zoxide
TIL that you can also use it like cd!
$ z /foo/bar
$ z ..
Writing tests in the traditional red-green approach is invaluable. But make sure they're red for the right reason!</just-lost-10-minutes>
If you haven't played with emacs-refactor yet: https://github.com/chrisbarrett/emacs-refactor it's well worth your time. E.g. emr-el-extract-to-let is invaluable.