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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 handle_subscribe(self, request):
 # type: (Subscribe) -> CallbackResponses """Handle a Subscribe request from outside. Called with lock taken"""

        ret = self._tree.handle_subscribe(request, request.path[1:])
        self._subscription_keys[request.generate_key()] = request
        return ret