Hint Resolve check_rval_wt check_lval_wt: ty.
Hint Extern 1 (wt_expr _ _ _) => (unfold wt_expr; simpl): ty.

Lemma ecall_sound:
forall fn args a, ecall fn args = OK a -> wt_expr ce e fn -> wt_exprlist ce e args -> wt_expr ce e a.