match hj : this ▸ j with
| head =>
  rw [←hj]
| tail j =>
  rw [eqNdrec_symm] at hj
  rw [hj, val_ndrec, val_tail] at h
  absurd ha
  apply Logic.Any.of_index ((j.undedup s).map (s.r x))
  rw [val_map, val_undedup]
  exact 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 x j.val
ha : ¬Logic.Any (List.map (Setoid.r x) xs)
this : List.dedup s (x :: xs) = x :: List.dedup s xs
⊢ head = ⋯ ▸ j
no goals