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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 add_404_page(app):
 """Build an extra ``404.html`` page if no ``"404"`` key is in the ``html_additional_pages`` config. """

    is_epub = isinstance(app.builder, EpubBuilder)
    config_pages = app.config.html_additional_pages

    if not is_epub and "404" not in config_pages:
        yield ("404", {}, "404.html")