A discussion of Forth vs Lisp macros leads to this extraordinary Forth code: https://github.com/davazp/eulex/blob/master/kernel/keyboard.fs#L135
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Over a sufficiently long time horizon, all code you write is legacy code.
I'm intrigued to see that Google has quantified that new code is generally buggier and less secure than code that has existed in your codebase for longer: https://security.googleblog.com/2024/09/eliminating-memory-safety-vulnerabilities-Android.html
One interesting design choice in Emacs that I haven't seen in other editors: reserved shortcuts.
An Emacs extension shouldn't use F5 through F12, or Ctrl-c LETTER. This lets users configure their own shortcuts. https://www.gnu.org/software/emacs/manual/html_node/emacs/Key-Bindings.html
Does it exist elsewhere? I miss it in VS Code, where e.g. all the Fn keys are already assigned.