docs: private ORCLUSCluster union(Relation<V> relation, ORCLUSCluster c1, ORCLUSCluster c2, int dim) {
      ORCLUSCluster c = new ORCLUSCluster();
      c.objectIDs = DBIDUtil.newHashSet(c1.objectIDs);
      c.objectIDs.addDBIDs(c2.objectIDs);
      c.objectIDs = DBIDUtil.newArray(c.objectIDs);
  
      if(c.objectIDs.size() > 0) {
        c.centroid = Centroid.make(relation, c.objectIDs).getArrayRef();
        c.basis = findBasis(relation, c, dim);
      }
      else {
        c.centroid = timesEquals(plusEquals(c1.centroid, c2.centroid), .5);
        c.basis = identity(dim, c.centroid.length);
      }
      return c;
    }