unfold distinct in |- *.
intros.
simpl in |- *.
red in |- *; intro.
red in H; (apply (H _ _ H0); auto with algebra).
Qed.
Hint Resolve seq_set_seq_preserves_distinct: algebra.

Lemma seq_set_seq_respects_distinct :
forall (A : Setoid) (n : Nat) (v : seq n A),
distinct (seq_set_seq v) -> distinct v.