Corollary check_rval_wt:
  forall a x, wt_expr ce e a -> check_rval a = OK x -> wt_rvalue ce e a.Corollary check_lval_wt:
  forall a x, wt_expr ce e a -> check_lval a = OK x -> wt_lvalue ce e a.

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.