docs: 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;
    }