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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 _get_shortcut_prefix(self, user_agent, standart_prefix):
 """ Returns the shortcut prefix of browser. :param user_agent: The user agent of browser. :type user_agent: str :param standart_prefix: The default prefix. :type standart_prefix: str :return: The shortcut prefix of browser. :rtype: str """

        # pylint: disable=no-self-use

        if user_agent is not None:
            user_agent = user_agent.lower()
            opera = 'opera' in user_agent
            mac = 'mac' in user_agent
            konqueror = 'konqueror' in user_agent
            spoofer = 'spoofer' in user_agent
            safari = 'applewebkit' in user_agent
            windows = 'windows' in user_agent
            chrome = 'chrome' in user_agent
            firefox = (
                ('firefox' in user_agent)
                or ('minefield' in user_agent)
            )
            internet_explorer = (
                ('msie' in user_agent)
                or ('trident' in user_agent)
            )

            if opera:
                return 'SHIFT + ESC'
            elif chrome and mac and (not spoofer):
                return 'CTRL + OPTION'
            elif safari and (not windows) and (not spoofer):
                return 'CTRL + ALT'
            elif (not windows) and (safari or mac or konqueror):
                return 'CTRL'
            elif firefox:
                return 'ALT + SHIFT'
            elif chrome or internet_explorer:
                return 'ALT'
            return standart_prefix
        return standart_prefix