code: public IDescriptorResult getCachedDescriptorValue(IBond bond) {
          if (cachedDescriptorValues == null) return null;
          return (IDescriptorResult) cachedDescriptorValues.get(bond);
      }
docs: Returns the cached DescriptorValue for the given IBond.
  
  @param bond the IAtom for which the DescriptorValue is requested
  @return     null, if no DescriptorValue was cached for the given IBond