code: private void addFileToTransform(Map<Transformer,Map<String,Set<URI>>> gs, Transformer t, Source s) {
          Map<String,Set<URI>> fs = gs.get(t);
          if (fs == null) {
              fs = new HashMap<String,Set<URI>>();
              gs.put(t, fs);
          }
          Set<URI> ss = fs.get(s.pkg().name());
          if (ss == null) {
              ss = new HashSet<URI>();
              fs.put(s.pkg().name(), ss);
          }
          ss.add(s.file().toURI());
      }
docs: Store the source into the set of sources belonging to the given transform.