human: Given a node-centered hypergraph: <hypergraph>, where nodes represent papers and hyperedges represent sets of papers co-cited by a source paper.

Details of the hypergraph:
Center node: "practical aspects of the bitcoin system"
  Abstract: Digital payment schemes show an ever increasing importance. Out of the countless different schemes available this article focuses on the popular Bitcoin system. The authors provide a description of Bitcoin's unique technological basis and its accompanying ecosystem of users, miners, trading platforms and vendors. Furthermore, this article discusses Bitcoin's currency-like features and the first regulatory actions take in the European Union and in the United States of America.

Connected through hyperedge HE-1 (induced by source paper "of two minds multiple addresses and one history characterizing opinions knowledge and perceptions of bitcoin across groups"):
  with members:
    - "a first look at the usability of bitcoin key management"
    - "operational distributed regulation for bitcoin"
    - "nowcasting the bitcoin market with twitter signals"
    - "the miner s dilemma"
    - "majority is not enough bitcoin mining is vulnerable"
    - "on bitcoin and red balloons"

Connected through hyperedge HE-2 (induced by source paper "bitcoin and beyond exclusively informational money"):
  with members:
    - "the unreasonable fundamental incertitudes behind bitcoin mining"
    - "informaticology combining computer science data science and fiction science"
    - "formaleuros formalcoins and virtual monies"
    - "searching publications on operating systems"
    - "on the definition of a theoretical concept of an operating system"
    - "machine structure oriented control code logic"
    - "a static theory of promises"

Question: Please tell me which class the center node belongs to. The 40 classes are: cs.NA(Numerical Analysis), cs.MM(Multimedia), cs.LO(Logic in Computer Science), cs.CY(Computers and Society), cs.CR(Cryptography and Security), cs.DC(Distributed, Parallel, and Cluster Computing), cs.HC(Human-Computer Interaction), cs.CE(Computational Engineering, Finance, and Science), cs.NI(Networking and Internet Architecture), cs.CC(Computational Complexity), cs.AI(Artificial Intelligence), cs.MA(Multiagent Systems), cs.GL(General Literature), cs.NE(Neural and Evolutionary Computing), cs.SC(Symbolic Computation), cs.AR(Hardware Architecture), cs.CV(Computer Vision and Pattern Recognition), cs.GR(Graphics), cs.ET(Emerging Technologies), cs.SY(Systems and Control), cs.CG(Computational Geometry), cs.OH(Other Computer Science), cs.PL(Programming Languages), cs.SE(Software Engineering), cs.LG(Machine Learning), cs.SD(Sound), cs.SI(Social and Information Networks), cs.RO(Robotics), cs.IT(Information Theory), cs.PF(Performance), cs.CL(Computation and Language), cs.IR(Information Retrieval), cs.MS(Mathematical Software), cs.FL(Formal Languages and Automata Theory), cs.DS(Data Structures and Algorithms), cs.OS(Operating Systems), cs.GT(Computer Science and Game Theory), cs.DB(Databases), cs.DL(Digital Libraries), cs.DM(Discrete Mathematics). Directly output the class name.

gpt: cs.CY(Computers and Society)