α : Type ?u
x✝ : ArrayBuffer α
cap : Nat
backing : Fin cap → Option α
size : Nat
h_size : size ≤ cap
h_full : ∀ (i : Nat), i < size → Option.isSome (backing { val := i, isLt := (_ : i < cap) }) = true
i : Nat
h : i < size
⊢ Option.isSome
      (if h_1 : i < cap then backing { val := i, isLt := (_ : { val := i, isLt := (_ : i < cap + cap) }.val < cap) }
      else none) =
    true