End EvalDo.
Section EvalReturn.
Variables (A B : Type).

Lemma try_write s1 s2 (v: A) (w : C) x i (r : cont B) :
verify (s1 tt) (x :-> v :+ i) r ->
verify (try_s (write_s x v) s1 s2) (x :-> w :+ i) r.