def command_line_parser(line):
    """
    Parses lines of the following form:

        Command line: ['to_html.py']

    :param line: sequence of lines
    :return: string containing the command_line, e.g., 'to_html.py'
    """
    idx_start, idx_end = line.find("['"), line.find("']")

    if idx_start == -1 or idx_end == -1:
        return ''
    else:
        return line[idx_start+2:idx_end]