docs: def _filename_in_headerblock(relative_path, contents, linter_options):
      """Check for a filename in a header block.
  
      like such:
      # /path/to/filename
      """
      del linter_options
      check_index = 0
  
      if len(contents) > 0:
          if _line_is_shebang(contents[0]):
              check_index = 1
  
      if len(contents) < check_index + 1:
          description = ("""Document cannot have less than """
                         """{0} lines""").format(check_index + 1)
          return LinterFailure(description, 1, replacement=None)
  
      header_path = relative_path.replace("\\", "/")
      regex = re.compile(r"^{0} \/{1}$".format(_HDR_COMMENT,
                                               re.escape(header_path)))
      if not regex.match(contents[check_index]):
          description = ("""The filename /{0} must be the """
                         """first line of the header""")
          return LinterFailure(description.format(header_path),
                               check_index + 1,
                               _comment_type_from_line(contents[check_index]) +
                               "/{0}\n".format(header_path))