/// <summary>
/// Add a task that will return a value.
/// </summary>
/// <typeparam name="T">The type of the return value.</typeparam>
/// <param name="callback">The task to execute.</param>
/// <returns>The return value of the task embedded in a Result object.</returns>
public Result <T> Call <T>(Func <T> callback)
{
Task task = Task.Create(callback);
reactor.Add(task, false);
return((Result <T>)task.result);
}