<cl> (setq a '(1 2 3 4)) (1 2 3 4) <cl> (defun nth-replace (nth data sexp) (rplaca (nthcdr nth sexp) data) sexp) NTH-REPLACE <cl> (nth-replace 0 'a a) (A 2 3 4) <cl> a (A 2 3 4) <cl> lll ((10 c) c d e) (nth-replace 1 20 lll) ((10 c) 20 d e) lll ((10 c) 20 d e) (nth-delete 1 lll) ((10 c) d e) lll ((10 c) d e) (nth-insert 1 30 lll) ((10 c) 30 d e) lll ((10 c) 30 d e)