On Sep 8, 2009, at 10:08 AM, Sam Johnston wrote:
This seems like a common purpose and I am glad you are discussing this on the ML.
The draft-divilly-atom-hierarchy is aimed at strengthening the current registration of 'up' to encourage unrelated uses to use other values. As you may know the use of multiple link@rel attribute values is prohibited in Atom, and, hence, is not relevant to Atom. I do understand that some formats that do allow these kind of multiple valued attributes.
In any case, it would be good to discuss this now considering that Link Header  is in Last Call and proposes to take over Atom's link registry and HTML5 has also proposed a link registry. As it pertains to Atom, we have had a spirited discussion about the meaning and use of up and down. We could also call it 'asc' and 'desc' in case there is difference of opinion about the use of up and down in HTML as opposed to other formats.
You can look at the archives of this and the atom-protocol ML to see the various options we have considered for dealing with various levels of hierarchy. An approach we have favored to deal with multiple levels of hierarchy is a combination of hierarchy links "up" and "down" and inline representation. That allows one to expand as many levels as they like while retaining the exact relationship between entries and avoiding an infinite number of rel values.
The term collection is already used in Atom by Atom Publishing Protocol [RFC5023].