Ranges
------

   GNUS introduced a concept that I found so useful that I've started
using it a lot and have elaborated on it greatly.

   The question is simple: If you have a large amount of objects that
are identified by numbers (say, articles, to take a *wild* example)
that you want to callify as being "included", a normal sequence isn't
very useful.  (A 200,000 length sequence is a bit long-winded.)

   The solution is as simple as the question: You just collapse the
sequence.

     (1 2 3 4 5 6 10 11 12)

   is transformed into

     ((1 . 6) (10 . 12))

   To avoid having those nasty `(13 . 13)' elements to denote a
lonesome object, a `13' is a valid element:

     ((1 . 6) 7 (10 . 12))

   This means that comparing two ranges to find out whether they are
equal is slightly tricky:

     ((1 . 5) 7 8 (10 . 12))

   and

     ((1 . 5) (7 . 8) (10 . 12))

   are equal.  In fact, any non-descending list is a range:

     (1 2 3 4 5)

   is a perfectly valid range, although a pretty long-winded one.  This
is also legal:

     (1 . 5)

   and is equal to the previous range.

   Here's a BNF definition of ranges.  Of course, one must remember the
semantic requirement that the numbers are non-descending.  (Any number
of repetition of the same number is allowed, but apt to disappear in
range handling.)

     range           = simple-range / normal-range
     simple-range    = "(" number " . " number ")"
     normal-range    = "(" start-contents ")"
     contents        = "" / simple-range *[ " " contents ] /
                       number *[ " " contents ]

   Gnus currently uses ranges to keep track of read articles and article
marks.  I plan on implementing a number of range operators in C if The
Powers That Be are willing to let me.  (I haven't asked yet, because I
need to do some more thinking on what operators I need to make life
totally range-based without ever having to convert back to normal
sequences.)