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(emacs-lisp-intro.info)Accumulate

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Recursive Pattern: _accumulate_
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   Another recursive pattern is called the `accumulate' pattern.  In
the `accumulate' recursive pattern, an action is performed on every
element of a list and the result of that action is accumulated with the
results of performing the action on the other elements.

   This is very like the `every' pattern using `cons', except that
`cons' is not used, but some other combiner.

   The pattern is:

   * If a list be empty, return zero or some other constant.

   * Else, act on the beginning of the list (the CAR of the list),
        -     and combine that acted-on element, using `+' or     some
          other combining function, with

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

   Here is an example:

     (defun add-elements (numbers-list)
       "Add the elements of NUMBERS-LIST together."
       (if (not numbers-list)
           0
         (+ (car numbers-list) (add-elements (cdr numbers-list)))))
     
     (add-elements '(1 2 3 4))
         => 10

   Note: Making a List of Files, for an example of the
accumulate pattern.