It looks like the "Converting Your First Function" has been written with clinic-generated C code interspersed with user C code. But it looks like nowadays a `clinic/{}.c.h` file is generated, so the "Converting Your First Function" should tell us to add the `#include "clinic/…`.

Also, the documentation states:

> If any of these items differ in any way, adjust your Argument
> Clinic function specification and rerun Tools/clinic/clinic.py
> until they are the same.

But since `METH_FASTCALL` it may be wrong (Clinic generated `METH_FASTCALL` but it was `METH_VARARGS|METH_KEYWORDS`).