Fixpoint Qeval_expr (env: PolEnv Q) (e: PExpr Q) : Q :=
  match e with
    | PEc c =>  c
    | PEX j =>  env j
    | PEadd pe1 pe2 => (Qeval_expr env pe1) + (Qeval_expr env pe2)
    | PEsub pe1 pe2 => (Qeval_expr env pe1) - (Qeval_expr env pe2)
    | PEmul pe1 pe2 => (Qeval_expr env pe1) * (Qeval_expr env pe2)
    | PEopp pe1 => - (Qeval_expr env pe1)
    | PEpow pe1 n => Qpower (Qeval_expr env pe1)  (Z.of_N n)
  end.

Lemma Qeval_expr_simpl : forall env e, Qeval_expr env e = match e with | PEc c => c | PEX j => env j | PEadd pe1 pe2 => (Qeval_expr env pe1) + (Qeval_expr env pe2) | PEsub pe1 pe2 => (Qeval_expr env pe1) - (Qeval_expr env pe2) | PEmul pe1 pe2 => (Qeval_expr env pe1) * (Qeval_expr env pe2) | PEopp pe1 => - (Qeval_expr env pe1) | PEpow pe1 n => Qpower (Qeval_expr env pe1) (Z.of_N n) end.