code: public CLIQUESubspace join(CLIQUESubspace other, double all, double tau) {
      long[] dimensions = joinLastDimensions(other);
      if(dimensions == null) {
        return null;
      }
  
      CLIQUESubspace s = new CLIQUESubspace(dimensions);
      for(CLIQUEUnit u1 : this.denseUnits) {
        for(CLIQUEUnit u2 : other.denseUnits) {
          CLIQUEUnit u = u1.join(u2, all, tau);
          if(u != null) {
            s.addDenseUnit(u);
          }
        }
      }
      if(s.denseUnits.isEmpty()) {
        return null;
      }
      return s;
    }
docs: Joins this subspace and its dense units with the specified subspace and its
  dense units. The join is only successful if both subspaces have the first
  k-1 dimensions in common (where k is the number of dimensions) and the last
  dimension of this subspace is less than the last dimension of the specified
  subspace.
  
  @param other the subspace to join
  @param all the overall number of feature vectors
  @param tau the density threshold for the selectivity of a unit
  @return the join of this subspace with the specified subspace if the join
  condition is fulfilled, null otherwise.
  @see de.lmu.ifi.dbs.elki.data.Subspace#joinLastDimensions