(define-struct abc (a b))


(define (foo p)
  (cond
    [(and (abc? p) (list? (abc-b p))) (cons (abc-a p) (list (abc-b p)))]
    )
  )

(define (bar1 lst)
(cond
  [(empty? lst) empty]
  [(abc? (first lst)) (cons (foo (first lst)) (bar1 (rest lst)))]
  [else (bar1 (rest lst))])
)


(bar1 (list (make-abc 4 (list 1 2 3))))