Emacs' undo facilities were ported to pure lisp in 2012. The editor actually gets more introspectable over time!
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I am super impressed that tree-sitter has built-in testing facilities for both parse trees: https://github.com/Wilfred/tree-sitter-elisp/blob/main/test/corpus/function_definition.txt
and syntax highlighting: https://github.com/Wilfred/tree-sitter-elisp/blob/main/test/highlight/functions.el
Syntax parsing is all about the tricky cases and this is a huge help.
How to take a screenshot in the 80s: https://www.kmjn.org/snippets/wilson85_screenshot.html
How things have changed! That process is hilariously complex compared with today's screenshot facilities.
It's a shame that app stores don't integrate bug reporting tools. Users end up using the review facilities instead.