Indeed, I've updated the documentation
elemsof() is implemented as a macro is C, but just the same as sizeof() in C2 (since this macro is implemented in so many programs).
There are just a few of these specials, so I don't think every C2 programmer will know them, just like sizeof() in C. Instead of a keyword
we could place them inside the c2 module, so you can access them with c2.sizeof() or sizeof() (if you import c2 local).