哪里"折叠" LINQ扩展方法？

我发现的LINQ的样本一个名为折叠整​​齐的方法（），我要使用。他们的例子：

I found in MSDN's Linq samples a neat method called Fold() that I want to use. Their example:

double[] doubles = { 1.7, 2.3, 1.9, 4.1, 2.9 }; 
double product = 
     doubles.Fold((runningProduct, nextFactor) => runningProduct * nextFactor);

不幸的是，我不能得到这个编译，无论是在他们的榜样还是在我自己的代码，我不能找到其他地方在MSDN（如可枚举或阵列扩展方法）中提到这种方法。我得到的错误是一个普通的老不知道呢错误：

Unfortunately, I can't get this to compile, either in their example or in my own code, and I can't find anywhere else in MSDN (like Enumerable or Array extension methods) that mention this method. The error I get is a plain old "don't know anything about that" error:

error CS1061: 'System.Array' does not contain a definition for 'Fold' and no 
extension method 'Fold' accepting a first argument of type 'System.Array' could 
be found (are you missing a using directive or an assembly reference?)

我用其他的方法，我相信来自LINQ的（如选择（）和Where （）），而我的使用System.Linq的，所以我认为这是一切OK。

I'm using other methods which I believe come from Linq (like Select() and Where()), and I'm "using System.Linq", so I think that's all OK.

请问这个方法是否真的存在在C＃3.5，如果是这样，我究竟做错了什么？

Does this method really exist in C# 3.5, and if so, what am I doing wrong?