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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 parse(input_: Union[str, FileStream], source: str) -> Optional[str]: """Parse the text in infile and save the results in outfile :param input_: string or stream to parse :param source: source name for python file header :return: python text if successful """


    # Step 1: Tokenize the input stream
    error_listener = ParseErrorListener()
    if not isinstance(input_, FileStream):
        input_ = InputStream(input_)
    lexer = jsgLexer(input_)
    lexer.addErrorListener(error_listener)
    tokens = CommonTokenStream(lexer)
    tokens.fill()
    if error_listener.n_errors:
        return None

    # Step 2: Generate the parse tree
    parser = jsgParser(tokens)
    parser.addErrorListener(error_listener)
    parse_tree = parser.doc()
    if error_listener.n_errors:
        return None

    # Step 3: Transform the results the results
    parser = JSGDocParser()
    parser.visit(parse_tree)

    if parser.undefined_tokens():
        for tkn in parser.undefined_tokens():
            print("Undefined token: " + tkn)
        return None

    return parser.as_python(source)