code: private Set<Source> calculateAddedSources() {
          Set<Source> added = new HashSet<>();
          for (String src : now.sources().keySet()) {
              if (prev.sources().get(src) == null) {
                  added.add(now.sources().get(src));
              }
          }
          return added;
      }
docs: Return those files belonging to now, but not prev.