docs: def endofcell_marker(source, comment):
      """Issues #31 #38:  does the cell contain a blank line? In that case
      we add an end-of-cell marker"""
      endofcell = '-'
      while True:
          endofcell_re = re.compile(r'^{}( )'.format(comment) + endofcell + r'\s*$')
          if list(filter(endofcell_re.match, source)):
              endofcell = endofcell + '-'
          else:
              return endofcell