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 qualify 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))
((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 valid:
(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.)