((S
  (Exp (Add (Exp (Id . "x")) (Exp (Int . "4"))))
  (S
   (Exp
    (Let
     (Id . "x")
     (Exp (Int . "1"))
     (Exp
      (Add
       (Exp (Add (Exp (Id . "x")) (Exp (Int . "5"))))
       (Exp (Int . "2"))))))
   (S
    (Exp
     (Let
      (Id . "y")
      (Exp
       (Add
        (Exp (Add (Exp (Id . "x")) (Exp (Int . "23"))))
        (Exp (Int . "1"))))
      (Exp
       (Add
        (Exp (Add (Exp (Id . "y")) (Exp (Id . "x"))))
        (Exp (Id . "x"))))))))))