code: func (f *filter) Zones(zones map[string]string) map[string]string {
  	result := map[string]string{}
  	for zoneID, zoneName := range zones {
  		if strings.HasSuffix(zoneName, f.domain) {
  			result[zoneID] = zoneName
  		}
  	}
  	return result
  }
docs: // Zones filters map[zoneID]zoneName for names having f.domain as suffix