have npos := calc 0 < 2 := by norm_num
                  _ ≤ n := hlte
case inr
n : ℕ
hn : a' n = 0
hlte : 2 ≤ n
n1 : ℕ := 2 * (n - 1) + 1
⊢ ∃ M, n < M ∧ a' M = 0
case inr
n : ℕ
hn : a' n = 0
hlte : 2 ≤ n
n1 : ℕ := 2 * (n - 1) + 1
npos : 0 < n
⊢ ∃ M, n < M ∧ a' M = 0