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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 selenium(self):
 """Get the instance of webdriver, it starts the browser if the webdriver is not yet instantied :return: a `selenium instance <http://selenium-python.readthedocs.org/ api.html#module-selenium.webdriver.remote.webdriver>` """

        if not self._web_driver:
            self._web_driver = self._start_driver()
        return self._web_driver