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: "saturation of concurrent collapsible pushdown systems"
  Abstract: Multi-stack pushdown systems are a well-studied model of concurrent computation using threads with first-order procedure calls. While, in general, reachability is undecidable, there are numerous restrictions on stack behaviour that lead to decidability. To model higher-order procedures calls, a generalisation of pushdown stacks called collapsible pushdown stacks are required. Reachability problems for multi-stack collapsible pushdown systems have been little studied. Here, we study ordered, phase-bounded and scope-bounded multi-stack collapsible pushdown systems using saturation techniques, showing decidability of control state reachability and giving a regular representation of all configurations that can reach a given control state.

Connected through hyperedge HE-1 (induced by source paper "ordered tree pushdown systems"):
  with members:
    - "saturation algorithms for model checking pushdown systems"
    - "model checking of ordered multi pushdown automata"
    - "strictness of the collapsible pushdown hierarchy"
    - "the safe lambda calculus"
    - "a finite semantics of simply typed lambda terms for infinite runs of automata"

Connected through hyperedge HE-2 (induced by source paper "unboundedness and downward closures of higher order pushdown automata"):
  with members:
    - "rewriting higher order stack trees"
    - "an approach to computing downward closures"
    - "senescent ground tree rewrite systems"

Connected through hyperedge HE-3 (induced by source paper "annotated stack trees"):
  with members:
    - "senescent ground tree rewrite systems"
    - "symbolic reachability analysis of higher order context free processes"
    - "the complexity of games on higher order pushdown automata"

Connected through hyperedge HE-4 (induced by source paper "saturation algorithms for model checking pushdown systems"):
  with members:
    - "modeling and verification of infinite systems with resources"
    - "model checking of ordered multi pushdown automata"
    - "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)