Given a list of list of integers, return the first item in each sublist, in a way that no duplication is occurred

answer.clj

(defn add-first-uniq [target coll]
  (cond (not (contains? target (first coll)))
        (conj target (first coll))
        
        :else
        (add-first-uniq target (rest coll))))

(reduce add-first-uniq
        #{}
        '((1 2 3) (1 2) (4 5)))

; => #{1 2 4}

lesson.clj

(facts
  (f '((1 4 5) (1 4 2) (10 4 1))) => '(1 4 10)
  (f '()) => '()
  (f '((1 2) () (3 4))) => (1 3))