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