If (member o l) finds o in the list l, it also returns the cdr of l beginning with o. This return value can be used, for example, to test for duplication. If o is duplicated in l, then it will also be found in the cdr of the list returned by member. This idiom is embodied in the next utility, duplicate:

>(duplicate ’a ’(a b c a d))
(A D)

(defun duplicate (obj lst &key (test #’eql))
    (member obj (cdr (member obj lst :test test))
            :test test))

— p.51

— On Lisp

— Paul Graham

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Exercise 4.4

Without using the existing function member, define duplicate as in

>(duplicate ’a ’(a b c a d))
(A D)

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— This answer is my guess. —

(defun my-member (obj lst)
    (cond ((not lst) NIL)
          ((eq obj (car lst)) lst)
          (t (my-member obj (cdr lst)))))

— This answer is my guess. —

— Me@2019-01-21 06:34:46 AM

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2019.01.21 Monday (c) All rights reserved by ACHK