code: def _filter_markdown(source, filters):
      
      lines = source.splitlines()
      # Filters is a list of 'hN' strings where 1 <= N <= 6.
      headers = [_replace_header_filter(filter) for filter in filters]
      lines = [line for line in lines if line.startswith(tuple(headers))]
      return '\n'.join(lines)
docs: Only keep some Markdown headers from a Markdown string.