NSKeyValueObservationInfo　をハクってみたけど、どうもできそうにないな。
そういう用途は想定外なんだろ。
ivars = (
　 "_retainCountMinusOne",
　 "_observances",
　 "_cachedHash",
　 "_cachedIsShareable",
　 "_observables"
);
methods = (
　 dealloc,
　 retain,
　 release,
　 description,
　 retainCount,
　 hash,
　 "isEqual:",
　 "_initWithObservances:count:"
);
properties = (
);