(let ((a (list 1 2 3))
      (b (list 4 5 6)))
     (cons a b))
; expect ((1 2 3) 4 5 6)

(let ((a 1)) (let ((b a)) (let ((c a)) (+ b c))))
; expect 2

(let ((x 1) (x 2) (y 3)) (+ x y))
; expect Error: duplicate symbol: x