specialize ih x_lt_n
case inl
x n : Nat
ih : ∀ (h : x < n), (Array.range n)[x] = x
h : x < n + 1
x_lt_n : x < n
⊢ (Array.range (n + 1))[x] = x
case inl
x n : Nat
h : x < n + 1
x_lt_n : x < n
ih : (Array.range n)[x] = x
⊢ (Array.range (n + 1))[x] = x