@usableFromInline
@derivative(of: append)
mutating func vjpAppend( element: Element) -> (
    value: Void, pullback: (inout TangentVector) -> Element.TangentVector
) {
    let appendedElementIndex = count
    append(element)
    return ((), { v in
        if v.base.isEmpty { return .zero }
        defer { v.base.removeLast() }
        return v.base[appendedElementIndex]
    })
}