sysl.c

A function with a C variadic tail (sum(n: int, ...)) reads that tail through these, exactly as a C function reads one through <stdarg.h>‘s macros. The compiler answers them itself – va_arg takes the type it reads from the position its value fills – so nothing here is a declaration. What the module gives them is a home: a file names the ones it uses, the way it names anything else it did not declare.

import sysl.c.{va_start, va_arg, va_end}

A file that declares a va_arg of its own calls that one, imported or not. The forms are specified in reference/ffi.md.