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: "on word and frontier languages of unsafe higher order grammars"
  Abstract: Higher-order grammars are extensions of regular and context-free grammars, where non-terminals may take parameters. They have been extensively studied in 1980's, and restudied recently in the context of model checking and program verification. We show that the class of unsafe order-(n+1) word languages coincides with the class of frontier languages of unsafe order-n tree languages. We use intersection types for transforming an order-(n+1) word grammar to a corresponding order-n tree grammar. The result has been proved for safe languages by Damm in 1982, but it has been open for unsafe languages, to our knowledge. Various known results on higher-order grammars can be obtained as almost immediate corollaries of our result.

Connected through hyperedge HE-1 (induced by source paper "intersection types for unboundedness problems"):
  with members:
    - "intersection types and counting"
    - "pumping lemma for higher order languages"
    - "the diagonal problem for higher order recursion schemes is decidable"
    - "relational semantics of linear logic and higher order model checking"
    - "ordered tree pushdown systems"
    - "finitary semantics of linear logic and higher order model checking"
    - "an approach to computing downward closures"
    - "regular cost functions part i logic and algebra over words"

Connected through hyperedge HE-2 (induced by source paper "the diagonal problem for higher order recursion schemes is decidable"):
  with members:
    - "unboundedness and downward closures of higher order pushdown automata"
    - "ordered tree pushdown systems"
    - "an approach to computing downward closures"
    - "using models to model check recursive schemes"
    - "a characterization for decidable separability by piecewise testable languages"
    - "finite automata for the sub and superword closure of cfls descriptional and computational complexity"
    - "computing downward closures for stacked counter automata"
    - "complexity of model checking recursion schemes for fragments of the modal mu calculus"

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.FL(Formal Languages and Automata Theory)