docs: def _build_line(colwidths, colaligns, linefmt):
      "Return a string which represents a horizontal line."
      if not linefmt:
          return None
      if hasattr(linefmt, "__call__"):
          return linefmt(colwidths, colaligns)
      else:
          begin, fill, sep,  end = linefmt
          cells = [fill*w for w in colwidths]
          return _build_simple_row(cells, (begin, sep, end))