This is a formal proof validator, which determines the correctness of Fitch-style natural deduction proofs ("Fitch proofs").
The tool is used by students who follow the course Introduction to Logic (for CS) at the University of Groningen (the Netherlands).
The tool also returns a (hopefully useful) error message in case the proof is not correct.
This application takes Fitch proofs as they are defined in Language, Proof and Logic, by Dave Barker-Plummer, Jon Barwise and John Etchemendy.
It is accessible here: https://fitch.themisrug.nl.
If you want to build and run the application locally, then clone the repository, install Cargo if you haven't already and install wasm-pack (to compile Rust to WebAssembly) and do:
wasm-pack build --target web
Once you have it compiled, open a server in the fitch-proof directory of the repository:
python3 -m http.server 8080
And then open http://localhost:8080/ in your favorite web browser.