http://gmailblog.blogspot.com/2010/04/new-in-labs-nested-labels-and-message.html

I think that this will be appreciated by those who like tags and thise who don't as well --given that, to the best of my knowledge, Gmail up to now did not provide folders or any way to organise one's messages hierarchically.

Yes, and the way to define a sub-label is easy and flexible - just add the parent label as the prefix separated by a forward slash

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Edwin Yip
Turn MS Word into a full-featured outliner software
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