/// <summary>
/// (I -> O)
/// </summary>
/// <typeparam name="TActivity"></typeparam>
/// <typeparam name="TAccept"></typeparam>
/// <typeparam name="TReturn"></typeparam>
/// <param name="builder"></param>
/// <returns></returns>
public static IActivityBuilder <TAccept, TReturn> When <TActivity, TAccept, TReturn>(Action <TActivity> builder = default)
where TActivity : IActivity <TAccept, TReturn>, new()
=> When <TActivity, TAccept, TReturn>(builder.Create());
public IActivityBuilder Then <TActivity>(Action <TActivity> builder = default)
where TActivity : IActivity, new()
=> Then(builder.Create());