proposition: Lemma None_cget T (r : loc T) e : nth_error (ofEnv e) (loc_id r) = None -> cget r e = fail.
proof: Proof. by move=> H; rewrite /cget H. Qed.
imports: JMeq mathcomp(all_ssreflect finmap) monae_lib HB(structures) hierarchy monad_lib fail_lib state_lib trace_lib monad_transformer monad_model