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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 rst_underline(heading: str, underline_char: str) -> str: """ Underlines a heading for RST files. Args: heading: text to underline underline_char: character to use Returns: underlined heading, over two lines (without a final terminating newline) """

    assert "\n" not in heading
    assert len(underline_char) == 1
    return heading + "\n" + (underline_char * len(heading))