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(emacs-lisp-intro.info)Keep

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Recursive Pattern: _keep_
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   A third recursive pattern is called the `keep' pattern.  In the
`keep' recursive pattern, each element of a list is tested; the element
is acted on and the results are kept only if the element meets a
criterion.

   Again, this is very like the `every' pattern, except the element is
skipped unless it meets a criterion.

   The pattern has three parts:

   * If a list be empty, return `nil'.

   * Else, if the beginning of the list (the CAR of the list) passes
         a test
        -     act on that element and combine it, using `cons' with

        -     a recursive call by the function on the rest (the CDR) of
          the list.

   * Otherwise, if the beginning of the list (the CAR of the list) fails
     the test
        -     skip on that element,

        -     and, recursively call the function on the rest (the CDR)
          of the list.

   Here is an example that uses `cond':

     (defun keep-three-letter-words (word-list)
       "Keep three letter words in WORD-LIST."
       (cond
        ;; First do-again-test: stop-condition
        ((not word-list) nil)
     
        ;; Second do-again-test: when to act
        ((eq 3 (length (symbol-name (car word-list))))
         ;; combine acted-on element with recursive call on shorter list
         (cons (car word-list) (keep-three-letter-words (cdr word-list))))
     
        ;; Third do-again-test: when to skip element;
        ;;   recursively call shorter list with next-step expression
        (t  (keep-three-letter-words (cdr word-list)))))
     
     (keep-three-letter-words '(one two three four five six))
         => (one two six)

   It goes without saying that you need not use `nil' as the test for
when to stop; and you can, of course, combine these patterns.