void checkNotFrozen(String classname) throws RuntimeException {
        CtClass clazz = getCached(classname);
        if (clazz == null) {
            if (!childFirstLookup && parent != null) {
                try {
                    clazz = parent.get0(classname, true);
                }
                catch (NotFoundException e) {}
                if (clazz != null)
                    throw new RuntimeException(classname
                            + " is in a parent ClassPool.  Use the parent.");
            }
        }
        else
            if (clazz.isFrozen())
                throw new RuntimeException(classname
                                        + ": frozen class (cannot edit)");
    }