In this document, Under Triggers, TriggerResult's action is incomplete
The description of PURGE is missing after and，As follows.
1) The first three decide how to act on their invocation event by returning a TriggerResult. The action can be one of the following:
- CONTINUE: do nothing,
- FIRE: trigger the computation,
- PURGE: clear the elements in the window, and
- FIRE_AND_PURGE: trigger the computation and clear the elements in the window afterwards.
Description in the source code
- All elements in the window are cleared and the window is discarded,
- without evaluating the window function or emitting any elements.