I've written my first nontrivial piece of Less and found it very pleasant. The ability to give names to metrics and style blocks is powerful
miniblog.
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GUI automation is a hard problem. I'm impressed by Hammerspoon (OS X) and its nontrivial examples in the basic docs:
Sorting, trigonometry, filesystem access, HTTP. All these libraries I took for granted until I created a programming language. Nontrivial!
Turns out that implementing a Common Lisp style condition system is nontrivial.