eval.c:1750
void
rb_frozen_class_p(klass)
    VALUE klass;
{
    char *desc = "something(?!)";

    if (OBJ_FROZEN(klass)) {
	if (FL_TEST(klass, FL_SINGLETON))
	    desc = "object";
	else {
	    switch (TYPE(klass)) {
	      case T_MODULE:
	      case T_ICLASS:
		desc = "module"; break;
	      case T_CLASS:
		desc = "class"; break;
	    }
	}
	rb_error_frozen(desc);
    }
}
