Using Core_kernel.List.mem. And why usingBase&Core instead of stdlib?

[RogerT]

Thanks for this URL.
This confirms that there is a real issue regarding the Ocaml Toolchain/dev environment. I suspected it without being able to explain it, and it’s much more important than I believed.

I notice the solution from @Leonidas:

@Leonidas

This is true. As a stopgap solution I recommend using clean switches per project and only install packages via dependencies declared in the opam file and avoiding to call opam install … directly.