proposition: Lemma flatten_eq : forall (A : Set)(ls1 ls2 : list (list A)), list_pred eq ls1 ls2 -> flatten ls1 = flatten ls2.
proof: Proof. induction 1; intuition; simpl in *; subst. f_equal. eauto. Qed.
imports: FCF.Rat List Permutation Arith FCF.EqDec FCF.StdNat Bool Permutation