docs: def _render_corpus_row(self, label, ngrams):
          """Returns the HTML for a corpus row."""
          row = ('<tr>\n<td>{label}</td>\n<td>{first}</td>\n<td>{only}</td>\n'
                 '<td>{last}</td>\n</tr>')
          cell_data = {'label': label}
          for period in ('first', 'only', 'last'):
              cell_data[period] = ', '.join(ngrams[label][period])
          return row.format(**cell_data)