> Int.abs can return a negative number.

The documentation for abs[1] explains: "Warning. This may be negative if the argument is Int.min_int."

I think that's just the nature of two's complement and not necessarily a fault of OCaml. C++ is also like that[2].

[1] https://v2.ocaml.org/api/Int.html

[2] https://gcc.godbolt.org/z/34b3GbvvT