Ltac destruct_in_members i m :=
  let H := fresh "H" in
  destruct (compute_in_members i m) eqn:H;
    [apply compute_in_members_true_iff in H |
     apply compute_in_members_false_iff in H].

Lemma in_members_dec: forall i m, {in_members i m} + {~ in_members i m}.