Emacs' undo facilities were ported to pure lisp in 2012. The editor actually gets more introspectable over time!
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Sometimes programming tools are so good that you miss them when using other languages. I see these mentioned the most frequently:
* IntelliJ (for Java)
* Slime+Emacs (for Common Lisp)
* Pharo (for Smalltalk)
I'm struck that they all have bespoke UIs.
Today I learnt that Emacs 28 shipped a context-menu-mode! https://oylenshpeegul.gitlab.io/blog/posts/20230129/
This seems quintessentially Emacs: deeply hackable, but building UI features in an order very different to the mainstream.
I've started keeping a list of particularly interesting bugs and patches that I've worked on: https://github.com/Wilfred/interesting-code
The time that I once removed *a single closing paren* in Emacs is still my favourite.