func (t *evTable) addExtraString(s string) extraStringID {
	if s == "" {
		return 0
	}
	if t.extraStringIDs == nil {
		t.extraStringIDs = make(map[string]extraStringID)
	}
	if id, ok := t.extraStringIDs[s]; ok {
		return id
	}
	t.nextExtra++
	id := t.nextExtra
	t.extraStrings = append(t.extraStrings, s)
	t.extraStringIDs[s] = id
	return id
}

// getExtraString returns the extra string for the provided ID.