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That's Clojure's EDN encoded for ClojureScript isn't it? I recognize this syntax from briefly working on a CLJ/CLJS app.

Nope, edn uses square brackets and doesn't have a `list` keyword. This is some custom representation.

Probably written in Common Lisp? Based on some quick searching I found this reference https://docs.ovh.com/us/en/web-paas/languages-lisp/

Or it could just be that s expressions are easy to parse.