Section Expr.
Context {typ : Type}
{func : Type}.
Context {RT : RType typ}
{T2 : Typ2 _ PreFun.Fun}
{RS : RSym func}.
Locate exprD'.
Check exprD'.
Instance Expr_expr : @Expr typ _ (@expr typ func) :=
{ exprD' := fun tus tvs e t => @exprD' _ _ _ _ _ tus tvs t e
; wf_Expr_acc := @wf_expr_acc typ func
(*  ; mentionsAny := mentionsAny *)
}.
Context {RTOk : RTypeOk}
{T2Ok : Typ2Ok T2}
{RSOk : RSymOk RS}.

Lemma exprD'_App_L_rw
: forall tus tvs td t e1 e2 e1D e2D,
typeof_expr tus tvs e1 = Some (typ2 td t) ->
exprD' tus tvs (typ2 td t) e1 = Some e1D ->
exprD' tus tvs td e2 = Some e2D ->
exprD' tus tvs t (App e1 e2) = Some (exprT_App e1D e2D).