Interface to content repository

• Distinction between content-item and content-revision
• Updates are revisioned
• Most attributes are stored in the revisions
• Content-items and content-revisions are acs-objects
• Content-items are named entries, unique per parent_id (e.g. folder)
  
• Extend attributes via subtyping/subclassing 
• Single XOTcl layer, accepting either item_ids or revision_ids