proposition: Lemma algebraic_append : algebraicity append_op.
proof: Proof. move=> A B f [t1 t2] /=. by rewrite 3!list_bindE -flatten_cat -map_cat. Qed.
imports: mathcomp(all_ssreflect) imonae_lib HB(structures) ihierarchy imonad_lib ifail_lib istate_lib itrace_lib imonad_transformer