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The Aldi-style insurgents who could be about to shake up the vets market
，更多细节参见有道翻译
The condition to break out of the loop is that our current frame is the outermost caller, at which point we access the returnValue of its child. The individual implementations for foldTree and foldForest will be responsible for changing which stack frame our frame variable points at in each step.，详情可参考谷歌
So this is my attempt at that.