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Perhaps less and less common but in various areas of scholarship printed material is still very important. User should be able to recognize that identifier is resolvable, be able to copy/type into browser and then get something useful. Brings in notions of recognizability and brevity/copyability
(e.g. long string of digits hard to copy accurately).
Web indexing
We will want resources identified and linked with PIDS to be indexed in popular search engines (Google, Yahoo!, msn, etc.). The URL that the resource is indexed under should be the PID and not the final server 
. What requirements for redirection etc. should be fulfilled?