let rec simplify e =
    match e with
    | CstI i -> CstI i
    | Var x -> Var x

    | Prim("+",e1, CstI 0 ) -> simplify e1
    | Prim("+",CstI 0, e2) -> simplify  e2
    | Prim("-",e1, CstI 0) -> simplify e1
    | Prim ("*",CstI 0, e2) -> CstI 0
    | Prim ("*",e1, CstI 0) -> CstI 0

    | Prim ("*",CstI 1, e2) -> simplify e2
    | Prim ("*",e1, CstI 1) -> simplify e1

    | Prim ("-", e1, e2) -> if simplify e1 = simplify e2 then CstI 0 else Prim("-", (simplify e1), (simplify e2))
    | Prim (a, b, c) -> Prim (a, (simplify b), (simplify c))