code: func NewExpiringCountIndex(resolution Meters, expiration Minutes) *CountIndex {
  	newExpiringCounter := func() interface{} {
  		return newExpiringCounter(expiration)
  	}
  
  	return &CountIndex{newGeoIndex(resolution, newExpiringCounter), make(map[string]Point)}
  }
docs: // NewExpiringCountIndex creates an index, which maintains an expiring counter for each cell.