How deep? They go all the way down the rabbit hole, through Wonderland, have a tea party, and come back out again, my friend. Here's one of my favorites:

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Cat Basis Purrsuit - http://www.oneweirdkerneltrick.com/catbasis.pdf

Abstract Meow miao mew meow mew meow mew mew meow miao meow meow mew meow meow miau mew miao meeeow meow, miau meow miao mew meeeeow mew miao miao miao. Meow miao mew meow mew (MMM), meow miao mew meow mew meow, meow meow miao meow state-of-the-art meow meow.