code: public boolean addConnectedUpstreamElementWithCheck( IHillSlope element ) {
          if (PfafstetterNumber.areConnectedUpstream(this.getPfafstetterNumber(), element.getPfafstetterNumber())) {
              if (!upstreamElements.contains(element)) {
                  upstreamElements.add(element);
                  element.addConnectedDownstreamElementWithCheck(this);
              }
              return true;
          }
          return false;
      }
docs: /* (non-Javadoc)
  @see org.hortonmachine.hmachine.modules.hydrogeomorphology.adige.core.IHillSlope#addConnectedUpstreamElementWithCheck(org.hortonmachine.hmachine.modules.hydrogeomorphology.adige.core.HillSlope)