struct Model {
	Network init_net;
	Stack<Network> nets;
	Stack<Scope> scopes;
	Network* cur_net = nets.top();
	Scope* cur_scope = scopes.top();
	Set<string> all_param_names;

	int name_counter{0};

	Place place {None};
	DeviceContext dev_ctx {None};
};

		def recurrent(callback):
			rnn = cur_net.add_op("recurrent");
			nets.push(rnn.step_net)
			cur_net = nets.top()
			# same thing for scope.
			callback()
			nets.pop()
			cur_net = nets.top()

		def step_net():
			fc(...)
			return fc(...)
		rnn = recurrent(step_net)