fact: Fixpoint is_inv_aux (l: list A) (a: A) {struct l}: bool := match l with nil => false | cons b l1 => if (A_dec (op a b) e) then if (A_dec (op b a) e) then true else is_inv_aux l1 a else is_inv_aux l1 a end.
imports: ZArith Tactic Wf_nat UList ListAux FGroup