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Staleness Processing State Continued - shunning objects and deleting certificates
We continue the discussion of State for processing stale certificates. There we introduced state and operations for incrementally adapting certificates to change of object content and identity. Here we introduce state and operations for facilitating incremental anticipation of object deletions.
In addition to the operations described in Staleness State, another operation is to indicate that some objects should be "shunned", which is simply a device for passing another set of objects to the certificate's reconsideration procedure, but the intention is that the reconsideration procedure should try to update the certificate content in such a way that it avoids reference to the "shunned" objects. The purpose of this operation is its use prior to deleting a collection of objects from the current closed map. When objects are deleted so are all objects that depend on them; by "shunning" the objects beforehand, the reconsideration procedure has a chance to rebuild the certificate to avoid referring to the objects whose deletion is to come, which would otherwise simply delete the certificate as well. Shunning an object thus makes any certificate referring simply to it stale, if it's not already, and adds itself to the "shunned object collection" for each of those certificates.
Finally, because reconsideration of a certificate may result in deletion of the certificate, and because we want to forestall deletion of such certificates until other certificates depending on them have had a chance to shun them, another part of the staleness processing state is a set of certificates marked for later deletion. When the reconsideration of a certificate requires its deletion, it is marked for deletion then shunned rather than being immediately deleted. When no stale certificates remain, certificates marked for deletion are deleted along with every object that refers to them. Of course, if any reconsideration procedures fail, then this point is never reached, and indeed this device could be exploited to protect an object from accidental deletion by referring to it with a certificate whose reconsideration procedure always fails.