public struct ModifierKeyTracker {
    private var isDown = false

    public init() {}

    /// Feed the current state of the bound modifier flag; returns an event only on a transition.
    public mutating func process(isDown: Bool) -> HotkeyEvent? {
        guard isDown != self.isDown else { return nil }
        self.isDown = isDown
        return isDown ? .pressed : .released
    }
}