ienumerable ienumerator

http://www.xue5.com/itedu/200802/105266.html

http://www.cnblogs.com/illele/archive/2008/04/21/1164696.html

http://codebetter.com/blogs/david.hayden/archive/2005/03/08/59419.aspx

http://blog.colinmackay.net/archive/2007/06/24/79.aspx