eval.c:8062
static void
safe_setter(val)
    VALUE val;
{
    int level = NUM2INT(val);

    if (level < ruby_safe_level) {
	rb_raise(rb_eSecurityError, "tried to downgrade safe level from %d to %d",
		 ruby_safe_level, level);
    }
    ruby_safe_level = level;
    curr_thread->safe = level;
}
