Wrote a flatten function. Witness!

gistfile1.rkt

(define (join-list lst1 lst2) #! Wrote my own append just because I could
  (define (append-item lst item)
    (if (eq? lst `())
        (list item)
        (cons (car lst) (append-item (cdr lst) item))))
  (if (eq? lst2 `())
      lst1
      (if (list? lst2)
          (join-list (append-item lst1 (car lst2)) (cdr lst2))
          (append-item lst1 lst2))))

(define (flatten slist)
  (if (eq? slist `())
      `()
      (if (list? (car slist))
          (join-list (flatten (car slist)) (flatten (cdr slist))) #! The key here is to use append instead of cons
          (cons (car slist) (flatten (cdr slist))))))

> (flatten `(a b c))
'(a b c)
> (flatten `((a) () (b ()) () (c)))
'(a b c)
> (flatten `((a b) c (((d)) e)))
'(a b c d e)
> (flatten `(a b (() (c))))
'(a b c)