If there is no mapping for t, or t has no events, returns
false.
If tag is neither NULL nor the empty string (""), outputs the
event sequence associated with t, plus one more event,
constructed from tag and event as described in ADD_EVENT.
Note: Due to these rules, you must create a mapping for a tree before you can add or output
events. Further, you need to add at least one event (by using
ADD_EVENT) before calling COMMIT_ERROR. In some
circumstances, you need to invent a dummy value and/or event for this purpose.