simp [length, Nat.add] at ih
case h.a
α : Type u_1
x₁ x₂ x x' : α
xs : List α
ih : length (x :: x' :: xs) ≤ 1 ∨ length (split (x :: x' :: xs)).fst < length (x :: x' :: xs)
⊢ length (split xs).fst + 1 < length xs + 3
case h.a
α : Type u_1
x₁ x₂ x x' : α
xs : List α
ih : length xs + 1 + 1 ≤ 1 ∨ length (split xs).fst + 1 < length xs + 1 + 1
⊢ length (split xs).fst + 1 < length xs + 3