RE: something like ring.el needed, but not circular

[Drew Adams]

> Is there in emacs a package that allows walking through
> a list in both directions, like ring.el, but which is NOT a 
> ring (I mean circular)?

Might one ask "why"? What do you have against rings? ;-)

If you mean that you want the behavior of, say, Emacs input-history
traversal: two directions, but two endpoints, then take a look at the
input-history code. Look for command `next-history-element' in library
simple.el.

Or you can use a ring and define an endpoint position on it - have your code
stop when it reaches that point from either direction.