private void printColumnHeads() {
        out.format("<thead>%n");
        out.format("  <tr class=%s>%n", COL_HEAD_CLASS);
        out.format("    <th class=%s>&#x25CA;</th>%n", COL_HEAD_CLASS);
        //noinspection UnusedDeclaration
        StringBuilder colName = new StringBuilder();
        for (int i = firstColumn; i < endColumn; i++) {
            colName.setLength(0);
            int cnum = i;
            do {
                colName.insert(0, (char) ('A' + cnum % 26));
                cnum /= 26;
            } while (cnum > 0);
            out.format("    <th class=%s>%s</th>%n", COL_HEAD_CLASS, colName);
        }
        out.format("  </tr>%n");
        out.format("</thead>%n");
    }