docs: def replace_u(matchobj):
      """Break the interval into parts, and replace 'u's.
  
      pieces - [pos/neg, start_year, start_month, start_day,
                pos/neg, end_year, end_month, end_day]
      """
      pieces = list(matchobj.groups(''))
      # Replace "u"s in start and end years.
      if 'u' in pieces[1]:
          pieces[1] = pieces[1].replace('u', '0')
      if 'u' in pieces[5]:
          pieces[5] = pieces[5].replace('u', '9')
      # Replace "u"s in start month.
      if 'u' in pieces[2]:
          pieces[2] = '-' + replace_u_start_month(pieces[2])
      # Replace "u"s in end month.
      if 'u' in pieces[6]:
          pieces[6] = '-' + replace_u_end_month(pieces[6])
      # Replace "u"s in start day.
      if 'u' in pieces[3]:
          pieces[3] = '-' + replace_u_start_day(pieces[3])
      # Replace "u"s in end day.
      if 'u' in pieces[7]:
          pieces[7] = '-' + replace_u_end_day(pieces[7], year=pieces[5],
                                              month=pieces[6])
      return ''.join((''.join(pieces[:4]), '/', ''.join(pieces[4:])))