Yep. Truly a mystery.
EDIT: Apparently their website design is just so poor their clickable links are identical to the non-clickable plain text. That link is a clickable word if you completely guess you can click on some of the apparent plain text.
IIUC a regular programming language with a certain set of restrictions already duals as a proof-assistant as discovered by Curry & Howard. By restrictions, I mean something like how Rust forces you to follow certain rules compared to Java.
Their proof assistant / programming language: https://en.m.wikipedia.org/wiki/Lean_(proof_assistant)
Some links here seem to be broken at the moment — and David's currently on vacation so they likely won't be fixed until January — but if you see for example https://lean-lang.org/basic-types/strings/ it's supposed to be https://lean-lang.org/doc/reference/latest/basic-types/strin...