((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"))))))))))