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: "ordered tree pushdown systems"
  Abstract: We define a new class of pushdown systems where the pushdown is a tree instead of a word. We allow a limited form of lookahead on the pushdown conforming to a certain ordering restriction, and we show that the resulting class enjoys a decidable reachability problem. This follows from a preservation of recognizability result for the backward reachability relation of such systems. As an application, we show that our simple model can encode several formalisms generalizing pushdown systems, such as ordered multi-pushdown systems, annotated higher-order pushdown systems, the Krivine machine, and ordered annotated multi-pushdown systems. In each case, our procedure yields tight complexity.

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"
    - "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 "recursion schemes the mso logic and the u quantifier"):
  with members:
    - "the diagonal problem for higher order recursion schemes is decidable"
    - "unboundedness and downward closures of higher order pushdown automata"
    - "an approach to computing downward closures"
    - "using models to model check recursive schemes"
    - "weak mso u with path quantifiers over infinite trees"
    - "io vs oi in higher order recursion schemes"
    - "weak mso with the unbounding quantifier"

Connected through hyperedge HE-3 (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"
    - "strictness of the collapsible pushdown hierarchy"

Connected through hyperedge HE-4 (induced by source paper "the diagonal problem for higher order recursion schemes is decidable"):
  with members:
    - "on word and frontier languages of unsafe higher order grammars"
    - "unboundedness and downward closures of higher order pushdown automata"
    - "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)