docs: public List<File> getInactiveOverlays() throws PatchingException {
          if (referencedOverlayDirectories == null) {
              walk();
          }
          List<File> inactiveDirs = null;
          for (Layer layer : installedIdentity.getLayers()) {
              final File overlaysDir = new File(layer.getDirectoryStructure().getModuleRoot(), Constants.OVERLAYS);
              final File[] inactiveLayerDirs = overlaysDir.listFiles(new FileFilter() {
                  @Override
                  public boolean accept(File pathname) {
                      return pathname.isDirectory() && !referencedOverlayDirectories.contains(pathname);
                  }
              });
              if (inactiveLayerDirs != null && inactiveLayerDirs.length > 0) {
                  if (inactiveDirs == null) {
                      inactiveDirs = new ArrayList<File>();
                  }
                  inactiveDirs.addAll(Arrays.asList(inactiveLayerDirs));
              }
          }
          return inactiveDirs == null ? Collections.<File>emptyList() : inactiveDirs;
      }