code: boolean add(Path filename) {
      synchronized (pendingReplications) {
        PendingInfo found = pendingReplications.get(filename);
        if (found == null) {
          pendingReplications.put(filename, new PendingInfo(filename));
          return true;
        }
        return false;
      }
    }
docs: Add a block to the list of pending Replications. Returns true
  if the filename is added for the first time.