apply Nat.succ_le_succ
case a
α : Type u_1
x₁ x₂ : α
xs : List α
ih : length (split xs).fst ≤ length xs
⊢ Nat.le (length (split xs).fst + 1) (length xs + 1)
case a.a
α : Type u_1
x₁ x₂ : α
xs : List α
ih : length (split xs).fst ≤ length xs
⊢ length (split xs).fst ≤ length xs