. decide
case zero

⊢ (Array.range 0).size = 0

case succ
n✝ : Nat
a✝ : (Array.range n✝).size = n✝
⊢ (Array.range (n✝ + 1)).size = n✝ + 1
case succ
n✝ : Nat
a✝ : (Array.range n✝).size = n✝
⊢ (Array.range (n✝ + 1)).size = n✝ + 1