'Schedule a new task to be executed at the given time.
This is "best-effort" scheduling and should only be used for coarse
synchronization. A task cannot be scheduled for multiple times
simultaneously; any previously scheduled instance of the same task
will be cancelled.
Arguments:
task (callable): task to be scheduled
at (float or int): epoch seconds when task should run
Returns:
Future: resolves to result of task call, or exception if raised'

def schedule(self, task, at):

    return self._delayed_tasks.add(task, at)
