error.c:321
static VALUE
exc_initialize(argc, argv, exc)
    int argc;
    VALUE *argv;
    VALUE exc;
{
    VALUE arg;

    if (rb_scan_args(argc, argv, "01", &arg) == 1) {
	VALUE mesg = arg;
	StringValue(mesg);	/* ensure mesg can be converted to String */
    }
    rb_iv_set(exc, "mesg", arg);
    rb_iv_set(exc, "bt", Qnil);

    return exc;
}
