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

Lemma ecomma_sound:
forall r1 r2 a, ecomma r1 r2 = OK a -> wt_expr ce e r1 -> wt_expr ce e r2 -> wt_expr ce e a.