final class Payload { }

final class LongOperationsChain {
    private func process() -> Future<Payload> {
        return startMainQueueOperation().then(backgroundQueueOperation(with:)).then(mainQueueOperation(with:))
    }

    private func startMainQueueOperation() -> Future<Payload> { return Promise<Payload>().future }
    private func backgroundQueueOperation(with payload: Payload) -> Payload { return payload }
    private func mainQueueOperation(with payload: Payload) -> Payload { return payload }

    private let backgroundQueue: DispatchQueue
}