// only to ExperimentEvent via an EventExperimental event.
	expData map[event.Experiment]*ExperimentalData
}

// addExtraString adds an extra string to the evTable and returns
// a unique ID for the string in the table.
func (t *evTable) addExtraString(s string) extraStringID {
	if s == "" {
		return 0
	}
	if t.extraStringIDs == nil {
		t.extraStringIDs = make(map[string]extraStringID)
	}