apply Nat.succ_lt_succ
case h
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 + 1 < length xs + 1 + 1 + 1 + 1
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