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: "strictness of the collapsible pushdown hierarchy"
  Abstract: We present a pumping lemma for each level of the collapsible pushdown graph hierarchy in analogy to the second author's pumping lemma for higher-order pushdown graphs (without collapse). Using this lemma, we give the first known examples that separate the levels of the collapsible pushdown graph hierarchy and of the collapsible pushdown tree hierarchy, i.e., the hierarchy of trees generated by higher-order recursion schemes. This confirms the open conjecture that higher orders allow one to generate more graphs and more trees.

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"
    - "on word and frontier languages of unsafe higher order grammars"
    - "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"

Connected through hyperedge HE-2 (induced by source paper "intersection types and counting"):
  with members:
    - "the diagonal problem for higher order recursion schemes is decidable"
    - "unboundedness and downward closures of higher order pushdown automata"
    - "ordered tree pushdown systems"

Connected through hyperedge HE-3 (induced by source paper "ordered tree pushdown systems"):
  with members:
    - "saturation algorithms for model checking pushdown systems"
    - "saturation of concurrent collapsible pushdown systems"
    - "model checking of ordered multi pushdown automata"
    - "the safe lambda calculus"
    - "a finite semantics of simply typed lambda terms for infinite runs of automata"

Connected through hyperedge HE-4 (induced by source paper "reachability in higher order counters"):
  with members:
    - "symbolic backwards reachability analysis for higher order pushdown systems"
    - "symbolic reachability analysis of higher order context free processes"

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)