I once (when ChatGPT first came out) launched into a conversation with ChatGPT using nothing but s-expressions. Didn't bother with a preamble, nor an explanation, just structured my prompt into a tree, forced said tree into an s-expression and hit enter.

I was very surprised to see that the response was in s-expressions too. It was incoherent, but the parens balanced at least.

Just tried it now and it doesn't seem to do that anymore.