[ CCode ( type_id = "dex_cancellable_get_type ()" ) ]
public sealed class Cancellable : Future
`DexCancellable` is a simple cancellation primitive which allows for you to create [class@Dex.
Future] that will reject upon cancellation.
Use this combined with other futures using [ctor@Dex.Future.all_race] to create a future that resolves when all other futures complete or [method@Dex.Cancellable.cancel] is called to reject.