Fixpoint map_PExpr (e : PExpr S) : PExpr C :=
  match e with
    | PEc c => PEc (C_of_S c)
    | PEX p => PEX p
    | PEadd e1 e2 => PEadd (map_PExpr e1) (map_PExpr e2)
    | PEsub e1 e2 => PEsub (map_PExpr e1) (map_PExpr e2)
    | PEmul e1 e2 => PEmul (map_PExpr e1) (map_PExpr e2)
    | PEopp e     => PEopp (map_PExpr e)
    | PEpow e n   => PEpow (map_PExpr e) n
  end.Definition map_Formula (f : Formula S)  : Formula C :=
  let (l,o,r) := f in
    Build_Formula (map_PExpr l) o (map_PExpr r).Definition eval_sexpr : PolEnv -> PExpr S -> R :=
  PEeval rplus rtimes rminus ropp phiS pow_phi rpow.Definition eval_sformula (env : PolEnv) (f : Formula S) : Prop :=
  let (lhs, op, rhs) := f in
    (eval_op2 op) (eval_sexpr env lhs) (eval_sexpr env rhs).

Lemma eval_pexprSC : forall env s, eval_sexpr env s = eval_pexpr env (map_PExpr s).