返回一个单值，它是这样构造的：首先以序列的前两个元素调用函数，再以返回值和第三个参数调用，依次执行下去。

"Reduce (func, sequence)" returns a single value constructed by calling the binary function func on the first two items of the sequence, then on the result and the next item, and so on.

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为什么需要支持单参数的构造函数和析构函数呀？

T need supply only a single-argument constructor and a destructor.

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