@Override
  public void writeExternal(Element element) throws WriteExternalException {
    PsiDocumentManager.getInstance(myProject).commitAllDocuments();

    if (myPsiElements.isEmpty() && myRangeMarkers.isEmpty() && mySerializedElements.isEmpty()) {
      throw new WriteExternalException();
    }

    if (mySerializedElements.isEmpty()) {
      for (SmartPsiElementPointer<PsiElement> ptr : myPsiElements) {
        PsiElement psiElement = ptr.getElement();
        if (psiElement == null || !psiElement.isValid()) {
          continue;
        }
        FoldingInfo fi = psiElement.getUserData(FOLDING_INFO_KEY);
        boolean state = fi != null && fi.expanded;
        String signature = FoldingPolicy.getSignature(psiElement);
        if (signature == null) {
          continue;
        }

        PsiFile containingFile = psiElement.getContainingFile();
        PsiElement restoredElement = FoldingPolicy.restoreBySignature(containingFile, signature);
        if (!psiElement.equals(restoredElement)) {
          StringBuilder trace = new StringBuilder();
          PsiElement restoredAgain =
              FoldingPolicy.restoreBySignature(containingFile, signature, trace);
          LOG.error(
              "element: "
                  + psiElement
                  + "("
                  + psiElement.getText()
                  + "); restoredElement: "
                  + restoredElement
                  + "; signature: '"
                  + signature
                  + "'; file: "
                  + containingFile
                  + "; injected: "
                  + InjectedLanguageManager.getInstance(myProject)
                      .isInjectedFragment(containingFile)
                  + "; languages: "
                  + containingFile.getViewProvider().getLanguages()
                  + "; restored again: "
                  + restoredAgain
                  + "; restore produces same results: "
                  + (restoredAgain == restoredElement)
                  + "; trace:\n"
                  + trace);
        }

        Element e = new Element(ELEMENT_TAG);
        e.setAttribute(SIGNATURE_ATT, signature);
        e.setAttribute(EXPANDED_ATT, Boolean.toString(state));
        element.addContent(e);
      }
    } else {
      // get back postponed state (before folding initialization)
      for (SerializedPsiElement entry : mySerializedElements) {
        Element e = new Element(ELEMENT_TAG);
        e.setAttribute(SIGNATURE_ATT, entry.mySerializedElement);
        e.setAttribute(EXPANDED_ATT, Boolean.toString(entry.myFoldingInfo.getExpanded()));
        element.addContent(e);
      }
    }
    String date = null;
    for (RangeMarker marker : myRangeMarkers) {
      FoldingInfo fi = marker.getUserData(FOLDING_INFO_KEY);
      boolean state = fi != null && fi.expanded;

      Element e = new Element(MARKER_TAG);
      if (date == null) {
        date = getTimeStamp();
      }
      if (date.isEmpty()) {
        continue;
      }

      e.setAttribute(DATE_ATT, date);
      e.setAttribute(EXPANDED_ATT, Boolean.toString(state));
      String signature =
          Integer.valueOf(marker.getStartOffset()) + ":" + Integer.valueOf(marker.getEndOffset());
      e.setAttribute(SIGNATURE_ATT, signature);
      String placeHolderText = fi == null ? DEFAULT_PLACEHOLDER : fi.placeHolder;
      e.setAttribute(PLACEHOLDER_ATT, placeHolderText);
      element.addContent(e);
    }
  }