docs: def add_line_if_absent(filename: str, line: str) -> None:
      """
      Adds a line (at the end) if it's not already in the file somewhere.
  
      Args:
          filename: filename to modify (in place)
          line: line to append (which must not have a newline in)
      """
      assert "\n" not in line
      if not is_line_in_file(filename, line):
          log.info("Appending line {!r} to file {!r}", line, filename)
          with open(filename, "a") as file:
              file.writelines([line])