First Draft of Atom Cross-posting Extension

I wrote up a first draft of an Atom extension for declaring cross-posting duplicates. It defines both a way for the primary version to declare the duplicates and a way for the duplicates to declare the primary version.

In practice it's very unlikely that the duplicates will declare the primary version, because most of the time they don't know they're duplicates and even if they did they probably wouldn't want to admit to being "just a copy". But I hope that publishers that create cross-post duplicates will see the benefit in declaring these in the feed to improve the usability of the feed when it's consumed into a multi-feed aggregator such as FriendFeed or MT Action Streams.