Abram--I stumbled upon equals-then assignment clauses when reading up on something else I'd needed to do with loop:

(loop repeat 5 for prev = 0 then i for i = 17 then (1+ i) collect (cons i prev)) ; => ((17 . 0) (18 . 17) (19 . 18) (20 . 19) (21 . 20))

note: these clauses are evaluated in order so this example wouldn't work if the equals-then clause for i appeared before the one for prev. if the prev clause comes first, it can take the value of i from the previous iteration before it's updated.

Justin