Ask HN: Examples of Beautiful Code Snippets?

I always enjoy looking at elegant or critical snippets of code that are at the heart of technologies that we use, and probably get called billions/trillions of times per day.

For example: Elixir defmacro: https://github.com/elixir-lang/elixir/blob/master/lib/elixir/lib/kernel.ex#L3941

Prepare signal in linux: https://elixir.bootlin.com/linux/latest/source/kernel/signal.c#L900

Do you have any examples you think are worth appreciating for a moment?