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: "piecewise testable tree languages"
  Abstract: This paper presents a decidable characterization of tree languages that can#N#be defined by a boolean combination of Sigma_1 sentences. This is a tree#N#extension of the Simon theorem, which says that a string language can be#N#defined by a boolean combination of Sigma_1 sentences if and only if its#N#syntactic monoid is J-trivial.

Connected through hyperedge HE-1 (induced by source paper "partially ordered automata and piecewise testability"):
  with members:
    - "the covering problem"
    - "complexity of detectability opacity and a diagnosability for modular discrete event systems"
    - "complexity of universality and related problems for partially ordered nfas"
    - "separability by piecewise testable languages is ptime complete"
    - "complexity of deciding detectability in discrete event systems"
    - "piecewise testable languages and nondeterministic automata"
    - "the height of piecewise testable languages and the complexity of the logic of subwords"
    - "on separation by locally testable and locally threshold testable languages"

Connected through hyperedge HE-2 (induced by source paper "wreath products of distributive forest algebras"):
  with members:
    - "going higher in the first order quantifier alternation hierarchy on words"
    - "an effective characterization of the alternation hierarchy in two variable logic"
    - "wreath products of forest algebras with applications to tree logics"
    - "a decidable characterization of locally testable tree languages"
    - "tree languages defined in first order logic with one quantifier alternation"

Connected through hyperedge HE-3 (induced by source paper "separability by piecewise testable languages is ptime complete"):
  with members:
    - "piecewise testable languages and nondeterministic automata"
    - "going higher in the first order quantifier alternation hierarchy on words"
    - "separating regular languages by piecewise testable and unambiguous languages"
    - "efficient separability of regular languages by subsequences and suffixes"
    - "around dot depth one"

Connected through hyperedge HE-4 (induced by source paper "piecewise testable languages and nondeterministic automata"):
  with members:
    - "on the index of simon s congruence for piecewise testability"
    - "on upper and lower bounds on the length of alternating towers"
    - "alternating towers and piecewise testable separators"
    - "separating regular languages by piecewise testable and unambiguous languages"
    - "efficient separability of regular languages by subsequences and suffixes"
    - "around dot depth one"

Connected through hyperedge HE-5 (induced by source paper "the height of piecewise testable languages and the complexity of the logic of subwords"):
  with members:
    - "piecewise testable languages and nondeterministic automata"
    - "on the state complexity of closures and interiors of regular languages with subwords and superwords"
    - "decidability in the logic of subsequences and supersequences"
    - "on the index of simon s congruence for piecewise testability"
    - "an approach to computing downward closures"
    - "the height of piecewise testable languages with applications in logical complexity"
    - "finite automata for the sub and superword closure of cfls descriptional and computational complexity"
    - "recompression a simple and powerful technique for word equations"

Connected through hyperedge HE-6 (induced by source paper "deciding definability in fo2 h v on trees"):
  with members:
    - "wreath products of forest algebras with applications to tree logics"
    - "a decidable characterization of locally testable tree languages"
    - "tree languages defined in first order logic with one quantifier alternation"
    - "two way unary temporal logic over trees"

Connected through hyperedge HE-7 (induced by source paper "a note on decidable separability by piecewise testable languages"):
  with members:
    - "on the state complexity of closures and interiors of regular languages with subwords and superwords"
    - "demystifying reachability in vector addition systems"
    - "an approach to computing downward closures"
    - "computing downward closures for stacked counter automata"
    - "the power of well structured systems"
    - "the separation problem for regular languages by piecewise testable languages"
    - "separating regular languages by piecewise testable and unambiguous languages"
    - "model checking vector addition systems with one zero test"

Connected through hyperedge HE-8 (induced by source paper "ef ex forest algebras"):
  with members:
    - "wreath products of forest algebras with applications to tree logics"

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)