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 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.