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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 instance(host=None, port=None):
 """ Singleton to return only one instance of Server. :returns: instance of Server """


        if not hasattr(WebServer, "_instance") or WebServer._instance is None:
            assert host is not None
            assert port is not None
            WebServer._instance = WebServer(host, port)
        return WebServer._instance