acgt.scm

(use srfi-1)

(define acgt '(#\a #\c #\g #\t))

(define (create n)
  (map list->string
       (let loop ((n n))
         (if (zero? n)
             (list '())
             (append-map
              (lambda (x)
                (map (lambda (y) (cons x y))
                     (loop (- n 1))))
              acgt)))))

(define (solve lst)
  (filter (lambda (str) (string-scan str "aag"))
          lst))

(solve (create 4))
;=> ("aaag" "aaga" "aagc" "aagg" "aagt" "caag" "gaag" "taag")