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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def command_line_runner():
 """ I run functions from the command-line! """

    filename = sys.argv[-1]
    if not filename.endswith(".rst"):
        print("ERROR! Please enter a ReStructuredText filename!")
        sys.exit()
    print(rst_to_json(file_opener(filename)))