public TimelineEvent<E> merge(TimelineEvent<E> event, Collection<TimelineEvent<E>> events) {
        return merge(event, events, null);
    }
Merge the given one event with the given collection of events without UI update. Only events within one group can be
merged. Note: after merging, the merged event will get the same properties as the given one event except start and end
dates.

@param event  given event to be merged with collection of events
@param events collection of events
@return TimelineEvent result event after merging
@throws IllegalStateException thrown if not all events are within the same group
primefaces/src/main/java/org/primefaces/model/timeline/TimelineModel.java