(define (RemoveElement element lst)
;if unique-left is applied to (list 1 4 2 1 5 4), the result would be (list 1 4 2 5)
  (cond
    [(empty? lst) empty]
    [(= element (first lst)) (RemoveElement element (rest lst))]
    [else (cons (first lst) (RemoveElement element (rest lst)))]))

(define (unique-left lst)
  (cond
    [(empty? lst) empty]
    [(member? (first lst) (rest lst)) (cons (first lst) (unique-left (RemoveElement (first lst) (rest lst))))]
    [else (cons (first lst) (unique-left (rest lst)))]
    )
  )

(define (unique-right lst)
;the result of applying it to (list 1 4 2 1 5 4) would be (list 2 1 5 4)
  (cond
    [(empty? lst) empty]
    [(not (member? (first lst) (rest lst))) (cons (first lst) (unique-right (rest lst)))]
    [else (unique-right (rest lst))]))