Maintain a set of greenlets that are still running.
Links to each item and removes it upon notification.
A variant of the apply() method which returns a Greenlet object.
If callback is specified then it should be a callable which accepts a single argument. When the result becomes ready callback is applied to it (unless the call failed).
A variant of the map() method which returns a Greenlet object.
If callback is specified then it should be a callable which accepts a single argument.