code: func toContainer(hostID int, idMap []IDMap) (int, error) {
  	if idMap == nil {
  		return hostID, nil
  	}
  	for _, m := range idMap {
  		if (hostID >= m.HostID) && (hostID <= (m.HostID + m.Size - 1)) {
  			contID := m.ContainerID + (hostID - m.HostID)
  			return contID, nil
  		}
  	}
  	return -1, fmt.Errorf("Host ID %d cannot be mapped to a container ID", hostID)
  }
docs: // toContainer takes an id mapping, and uses it to translate a
  // host ID to the remapped ID. If no map is provided, then the translation
  // assumes a 1-to-1 mapping and returns the passed in id