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stack: check if stack is empty more pythonically
Wed, 08 May 2019 16:00:34 +0800, by Anton Shestakov
stack: implement __bool__ and __nonzero__
Wed, 08 May 2019 15:57:54 +0800, by Anton Shestakov
stack: get stack data directly from stack and remove stackdata()
Sun, 05 May 2019 17:39:46 +0800, by Anton Shestakov

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