code: public Set<Location> getDerefLocationSet(ValueNumber vn) {
          Set<Location> derefLocationSet = derefLocationSetMap.get(vn);
          if (derefLocationSet == null) {
              derefLocationSet = new HashSet<>();
              derefLocationSetMap.put(vn, derefLocationSet);
          }
          return derefLocationSet;
      }
docs: Get the set of dereference Locations for given value number.
  
  @param vn
  the value number
  @return the set of dereference Locations