[Checker Dev] Get arg(0) of VAArgExpr ?

Hi all,

I write a checker and I use REGISTER_MAP_WITH_PROGRAMSTATE with
MemRegion* for identify which va_list is init/used/close.
But to check if __builtin_va_arg use an init va_list, I use
I check in documentation but I don't find how to getArg(0) of VAArgExpr.

So How to getArg(0) with VAArgExpr ?

Thanks for taking the time,