[NNTP] Article references

[Clive D.W. Feather]

Wording attempt two:

I propose modifying section 6 as follows:

   News reading clients have available a variety of mechanisms to
   retrieve articles via NNTP.  The news articles are stored and indexed
   using three types of keys.  One key is the message-id of an article.
   Another key is composed of the newsgroup name and the article number
   within that newsgroup.  That key MUST be unique to a particular
   server (there will be only one article with that number within a
   particular newsgroup), but is not required to be globally unique.
   Additionally, because the same article can be cross-posted to
   multiple newsgroups, there may be multiple keys that point to the
-  same article on the same server.  The final key is the arrival
+  same article on the same server; nevertheless, a given article
+  MUST NOT have two different article numbers in any particular
+  newsgroup on a particular server. The final key is the arrival
   timestamp, giving the time that the article arrived at the server.

-- 
Clive D.W. Feather  | Work:  <clive at demon.net>   | Tel:    +44 20 8495 6138
Internet Expert     | Home:  <clive at davros.org>  | Fax:    +44 870 051 9937
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