@Nullable
      @Override
      public Iterable<VirtualFile> getChildrenIterable(@NotNull VirtualFile file) {
        if (dbOnly) {
          return file.isDirectory()? ((NewVirtualFile)file).iterInDbChildren() : null;
        }
        if (file.equals(fromFile) || !file.isDirectory()) {
          return null; // skipping additional checks for the initial file and non-directory files
        }
        // optimization: for all files that are not under content of currently opened projects iterate over DB children
        return isInContentOfOpenedProject(file)? null : ((NewVirtualFile)file).iterInDbChildren();
      }
@Nullable
      @Override
      public Iterable<VirtualFile> getChildrenIterable(@NotNull VirtualFile file)