编译F＃多文件工程的文件顺序问题

学习F#时碰到一个奇怪的问题vs2015报错
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Files
in libraries or multiple-file applications must begin with a namespace or module declaration, e.g. 'namespace SomeNamespace.SubNamespace' or 'module SomeNamespace.SomeModule'

在网上搜索了一下。发现stack overflow有解答，摘录一下备忘点击打开链接

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Sounds
like this is an order-of-files-in-the-project issue. The last file is the entry point ("main method"), sounds like you have Alg.fs last, and you need Program.fs last. You can re-order them via the right-click context menu in VS Solution Explorer.

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简单说，就是多文件工程在VS中需要保证Program.fs放在最后，在VS的解决方案资源管理器中拖拽一下顺序。就可以了 

好不自然的约束啊。明显是偶发复杂性吗。怕忘，因此记录一下。