rw [←h, unsublist_pos_head]
case head.mp
α : Type u_1
p✝ : α → Prop
inst✝¹ : DecidablePred p✝
p : α → Prop
inst✝ : DecidablePred p
x : α
xs : List α
ih : ∀ (i : xs.Index) (hi : p i.val) (j : (List.sublist p xs).Index), sublist p i hi = j ↔ i = unsublist p j
j : (List.sublist p (x :: xs)).Index
hx : p x
hi : p head.val
h : ⋯ ▸ head = j
⊢ head = unsublist p j
no goals