symmetry
α : Type u_1
s : Setoid α
inst✝ : DecidableRel Setoid.r
xs✝ xs : List α
i : xs.Index
⊢ Setoid.r (undedup s (dedup s i)).val i.val
α : Type u_1
s : Setoid α
inst✝ : DecidableRel Setoid.r
xs✝ xs : List α
i : xs.Index
⊢ Setoid.r i.val (undedup s (dedup s i)).val