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 sifthenelse_sound: forall a s1 s2 s, sifthenelse a s1 s2 = OK s -> wt_expr ce e a -> wt_stmt ce e rt s1 -> wt_stmt ce e rt s2 -> wt_stmt ce e rt s.