docs: def emphasis_sub(match):
      """Substitutes <strong>, <em>, and <strong><em> tags."""
      level = len(match.group(1))
      content = match.group(2)
      if level == 3:
          return '<strong><em>{0}</em></strong>'.format(content)
      elif level == 2:
          return '<strong>{0}</strong>'.format(content)
      elif level == 1:
          return '<em>{0}</em>'.format(content)