TO-DO
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- Polish UI for node subscriptions
- Handling of mixed subscription types
- Better digest formatting
- Add automated tests for modules
- Support for message localization for multiple languages
- Delete subscriptions when they don't apply anymore (I.e. node deleted)

06/02/07 - 
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- Fixed issue with group name on form confirmation
- Implemented access permission for group subscriptions
- Added module field to allow other modules to handle data in notifications tables
- Added status field to allow disabled subscriptions on the table
- UI Type for nodes defaults to 'links'
- Added simpletest for basic API, detected a pair bugs on notifications_get_subscriptions :-)
- Added simpletest for content notifications. Needs more cases.
- Fixed buggy url generation (destination not working on cron)
- Improved page workflow and fixed bug in confirmation form (send_method)