proposition: Lemma naturality_empty : naturality [the functor of Empty.acto \o M] M empty.
proof: Proof. by move=> A B h; apply boolp.funext. 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