Bisect_ppx helps you test thoroughly. It is a small preprocessor that inserts instrumentation at places in your code, such as if-then-else and match expressions. After you run tests, Bisect_ppx gives a nice HTML report showing which places were visited and which were missed.

Usage is simple - add package bisect_ppx when building tests, run your tests, then run the Bisect_ppx report tool on the generated visitation files.