code: public void updateFragementWithSelectedCorpus(Set<String> corpora)
    {
      if (corpora != null && !corpora.isEmpty())
      {
        String fragment = "_c=" + Helper.encodeBase64URL(StringUtils.
          join(corpora, ","));
        UI.getCurrent().getPage().setUriFragment(fragment);
      }
      else
      {
        UI.getCurrent().getPage().setUriFragment("");
      }
    }
docs: Adds the _c fragement to the URL in the browser adress bar when a corpus is
  selected.
  
  @param corpora A list of corpora, which are add to the fragment.