open fun add(registeredFlowAction: RegisteredFlowAction): Unit = illegalState()

        open fun store(): Pair<Any, State> = illegalState()

        open fun load(memento: Any): State {
            return Loaded(memento.uncheckedCast())
        }

        protected

            cacheAction: String,
            requestedTasks: String?,
            totalProblemCount: Int
        ): Pair<State, File?> =
            illegalState()

        open fun close(): State =
            illegalState()

        private
        fun illegalState(): Nothing =
            throw IllegalStateException("Operation is not valid in ${javaClass.simpleName} state.")

        class Idle(