proposition: Lemma handleE : handle = (fun A m m' => if m is inr x then m else m').
proof: Proof. by apply funext_dep => A; apply funext => -[]. Qed.
imports: mathcomp(all_ssreflect) imonae_lib HB(structures) ihierarchy imonad_lib ifail_lib istate_lib itrace_lib imonad_transformer