code: public double getUpstreamArea( List<PfafstetterNumber> limit ) {
          if (hillslopeUpstreamArea == -1) {
  
              List<IHillSlope> basins = new ArrayList<IHillSlope>();
              getAllUpstreamElements(basins, limit);
  
              hillslopeUpstreamArea = 0;
              for( IHillSlope elementarBasin : basins ) {
                  hillslopeUpstreamArea = hillslopeUpstreamArea + elementarBasin.getHillslopeArea();
              }
          }
          return hillslopeUpstreamArea;
      }
docs: /* (non-Javadoc)
  @see org.hortonmachine.hmachine.modules.hydrogeomorphology.adige.core.IHillSlope#getUpstreamArea(java.util.List)