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

Lemma eassign_sound:
forall l r a, eassign l r = OK a -> wt_expr ce e l -> wt_expr ce e r -> wt_expr ce e a.