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}.