code: private String[] getAltIds(FedoraClient fedora, String vitalPid,
              String dsPid) {
          Datastream ds = getDatastream(fedora, vitalPid, dsPid);
          if (ds != null) {
              return ds.getAltIDs();
          }
          return new String[]{};
      }
docs: Find and return any alternate identifiers already in use in fedora for
  the given datastream.
  
  @param fedora An instantiated fedora client
  @param vitalPid The VITAL PID to use
  @param dsPid The datastream ID on the object
  @returns String[] An array or String identifiers, will be empty if
  datastream does not exist.