source: branches/typed-asm/abcl/contrib/asdf-install/split-sequence.lisp

;;;; SPLIT-SEQUENCE
;;;
;;; This code was based on Arthur Lemmens' in
;;; <URL:http://groups.google.com/groups?as_umsgid=39F36F1A.B8F19D20%40simplex.nl>;
;;;

(in-package #:asdf-install)

(defun split-sequence-if (predicate seq &key (count nil) (remove-empty-subseqs nil) (from-end nil) (start 0) (end nil) (key nil key-supplied))
  "Return a list of subsequences in seq delimited by items satisfying
predicate.

If :remove-empty-subseqs is NIL, empty subsequences will be included
in the result; otherwise they will be discarded.  All other keywords
work analogously to those for CL:SUBSTITUTE-IF.  In particular, the
behaviour of :from-end is possibly different from other versions of
this function; :from-end values of NIL and T are equivalent unless
:count is supplied. The second return value is an index suitable as an
argument to CL:SUBSEQ into the sequence indicating where processing
stopped."
  (let ((len (length seq))
        (other-keys (when key-supplied 
          (list :key key))))
    (unless end (setq end len))
    (if from-end
        (loop for right = end then left
              for left = (max (or (apply #'position-if predicate seq 
           :end right
           :from-end t
           other-keys)
          -1)
            (1- start))
              unless (and (= right (1+ left))
                          remove-empty-subseqs) ; empty subseq we don't want
              if (and count (>= nr-elts count))
              ;; We can't take any more. Return now.
              return (values (nreverse subseqs) right)
              else 
              collect (subseq seq (1+ left) right) into subseqs
              and sum 1 into nr-elts
              until (< left start)
              finally (return (values (nreverse subseqs) (1+ left))))
      (loop for left = start then (+ right 1)
            for right = (min (or (apply #'position-if predicate seq 
          :start left
          other-keys)
         len)
           end)
            unless (and (= right left) 
                        remove-empty-subseqs) ; empty subseq we don't want
            if (and count (>= nr-elts count))
            ;; We can't take any more. Return now.
            return (values subseqs left)
            else
            collect (subseq seq left right) into subseqs
            and sum 1 into nr-elts
            until (>= right end)
            finally (return (values subseqs right))))))