struct.c:199
VALUE
#ifdef HAVE_STDARG_PROTOTYPES
rb_struct_define(const char *name, ...)
#else
rb_struct_define(name, va_alist)
    const char *name;
    va_dcl
#endif
{
    va_list ar;
    VALUE nm, ary;
    char *mem;

    if (!name) nm = Qnil;
    else nm = rb_str_new2(name);
    ary = rb_ary_new();

    va_init_list(ar, name);
    while (mem = va_arg(ar, char*)) {
	ID slot = rb_intern(mem);
	rb_ary_push(ary, ID2SYM(slot));
    }
    va_end(ar);

    return make_struct(nm, ary, rb_cStruct);
}
