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Proposed deletion reviewing is the process of checking whether pages that have been deleted by way of the proposed deletion process were correctly deleted and, if, in the opinion of the reviewer, they were not, undeleting them.
The most logical way to perform this process seems to be to systematically examine the WP:PRODLIST from start to finish (or from finish to start) and then consider the merits of the deleted pages therein redlinked, which can be undeleted on request at WP:REFUND if it is decided their deletion was erroneous. Since expired PRODs are automatically redacted from the prodlist, this involves looking at old versions of the list in old revisions in the page history of the prodlist. (The first revision of the list is that of 13 July 2006).
The nature of the proposed deletion process is such that there may be a very large number of pages that ought to be undeleted. In view of the length of the prodlist, it would be desirable for a WikiProject to be set up for the purpose of systematically examining it.