docs: public void printTree(final Tree t, final String id, final PrintWriter pw) {
      final boolean inXml = propertyToBoolean(options, "xml");
      if (t == null) {
        // Parsing didn't succeed.
        if (inXml) {
          pw.print("<s");
          if (id != null && ! "".equals(id)) {
            pw.print(" id=\"" + XMLUtils.escapeXML(id) + '\"');
          }
          pw.println(" skipped=\"true\"/>");
          pw.println();
        } else {
          pw.println("SENTENCE_SKIPPED_OR_UNPARSABLE");
        }
      } else {
        if (inXml) {
          pw.print("<s");
          if (id != null && ! "".equals(id)) {
            pw.print(" id=\"" + XMLUtils.escapeXML(id) + '\"');
          }
          pw.println(">");
        }
        printTreeInternal(t, pw, inXml);
        if (inXml) {
          pw.println("</s>");
          pw.println();
        }
      }
    }