code: public Set<Location> getUnconditionalDerefLocationSet(ValueNumber vn) {
          Set<Location> derefLocationSet = derefLocationSetMap.get(vn);
          if (derefLocationSet == null) {
              derefLocationSet = Collections.<Location> emptySet();
          }
          return derefLocationSet;
      }
docs: Get the set of Locations where given value is guaranteed to be
  dereferenced. (I.e., if non-implicit-exception control paths are
  followed, one of these locations will be reached).
  
  @param vn
  the value
  @return set of Locations, one of which will definitely be reached if
  non-implicit-exception control paths are followed