"seen" marks create adjacent lists

[Karl Kleinpaste <karl@charcoal.com>]

I've been experimenting a bit with seeing what happens to .newsrc.eld
regarding certain parameters, so I've been saving and formatting
.newsrc.eld at various points, so as to diff them.

I've noticed that "seen" marks have a pronounced tendency to create
pointlessly adjacent number lists, rather than subsuming new items
into existing lists.  Here's an example:

@@ -1389,9 +1390,10 @@
            (90348 . 90351)))
          (nntp "news"))
         ("nntp+news:comp.protocols.dns.bind" 3
-         ((1 . 106775))
+         ((1 . 106777))
          ((seen
-           (105429 . 106775)))
+           (105429 . 106776)
+           106777))
          (nntp "news"))
         ("nntp+news:comp.protocols.dns.ops" 3
          ((1 . 93004))

Before this particular run of Gnus, I'd seen up through 106775.  When
I ran Gnus, there were 2 articles there, which I read (or at least
used `d' on), and Gnus correctly identifies that I've seen up through
106777 now.

But notice that the "seen" list has gone from a single cons up to
106775, to a cons up to 106776 *plus* a straggler 106777.

What's the point of that?  It seems to me that this should have
resulted in a single cons (105429 . 106777).  And in fact when I
hand-edit .newsrc.eld to reflect this, it works just fine.

In some very busy groups I read, this has induced some really
impressively large "seen" lists, all numerically adjacent.

--karl