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