依后见之明，使Point成为可变的这个决定被证明对于程序清晰性和性能是昂贵的代价。
In hindsight, the decision to make Point mutable turned out to be costly to program clarity and performance.
然而，本净以下所提供的版本并不正确，我已依圆明版的版本完成。
However, the version provided below by Ben Jing is not correct. I had done it based on the Yuan Ming Edition.
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