A mapping function that allows splicing lists into the result. If fun returns a list whose first element is @ the following elements will be spliced into the list.

gistfile1.txt

(defun mapcar@ (fun seq)
  (let (result)
    (loop for elem in (reverse seq)
          for newelem = (funcall fun elem)
          if (and (listp newelem)
                  (eq (car newelem) '@))
          do (loop for newelem2 in (cdr newelem)
                   do (setq result (cons newelem2 result)))
          else do (setq result (cons newelem result)))
    result))

An example usage would be great. :)

For example you could use it for counting subsequences:

(mapcar@ '(closure (acc prev) (b)
                   (cond b
                         ((eq prev nil) (setq prev b acc (cons b acc)))
                         ((eq prev b) (setq acc (cons b acc))
                          'omit)
                         (t (prog1 (length acc)
                              (setq acc nil prev b)))))
         seq)

(a a a b b b c d d d) => (3 2 1 3)

Actually, it would also need something to signal the end of the list, and I guess there is probably another mapping function that could do this, but it just seems the logical next step - ragged mapping.
Btw.. how do you get coloured syntax highlighting in your gist?

Actually forget it. Just realized it's probably easier to do it directly with a loop.