rw [val_head] at h
α : Type u_1
s : Setoid α
inst✝ : DecidableRel Setoid.r
xs✝ : List α
x : α
xs : List α
ih : ∀ {i : xs.Index} {j : (List.dedup s xs).Index}, Setoid.r i.val j.val → dedup s i = j
i : (x :: xs).Index
j : (List.dedup s (x :: xs)).Index
h : Setoid.r head.val j.val
⊢ dedup s head = j
α : Type u_1
s : Setoid α
inst✝ : DecidableRel Setoid.r
xs✝ : List α
x : α
xs : List α
ih : ∀ {i : xs.Index} {j : (List.dedup s xs).Index}, Setoid.r i.val j.val → dedup s i = j
i : (x :: xs).Index
j : (List.dedup s (x :: xs)).Index
h : Setoid.r x j.val
⊢ dedup s head = j