浅谈yield
http://www.cnblogs.com/qlb5626267/archive/2009/05/08/1452517.html
.NET中yield关键字的用法
http://blog.csdn.net/aspnet2002web/article/details/6083417
When you use the yield keyword in a statement, you indicate that the method, operator, or get accessor in which it appears is an iterator.
yield (C# Reference)
https://msdn.microsoft.com/en-us/library/9k7k7cf0.aspx
https://ayende.com/blog/3082/pipes-and-filters-the-ienumerable-appraoch
