Information and Computation special issue from TACS 2001

The fourth symposium on Theoretical Aspects of Computer Software (TACS 2001) was held in Sendai, Japan, on October 29–31, 2001. The program co-chairs were Naoki Kobayashi and Benjamin C. Pierce. The conference proceedings appeared as Lecture Notes in Computer Science, Volume 2215 (Springer-Verlag, 2001). The program committee received 59 papers and accepted 21. After the symposium, expanded versions of eight papers were solicited for this special issue of Information and Computation. After a standard journal refereeing process, seven were accepted, spanning a broad range of topics. The issue opens with an article by Andrew Pitts entitled “Nominal logic, a first-order theory of names and binding,” based on Pitts’ invited lecture at the symposium. This article introduces a first-order manysorted logic for reasoning about syntax involving variable binding. The most notable advantage of this logic over recent other proposals is its simplicity, achieved by choosing “swapping” and “freshness” as primitive constructs. Caires and Cardelli’s article, “A spatial logic for concurrency (part I)” is based on Cardelli’s invited lecture. It introduces a modal logic for specifying the behavior and spatial structure of concurrent systems. The most novel feature of the logic is a convincing treatment of properties of name restriction, obtained by adapting Gabbay and Pitts’ work on a semantics for freshness of names. “A linear time algorithm for monadic querying of indefinite data over linearly ordered domains,” by Ogawa, studies the problem of query answering over indefinite databases. It extends van der Meyden’s work, which provided a non-constructive proof of the linear time data complexity for evaluating fixed disjunctive monadic queries, by constructing a linear time algorithm based on the constructive proof of Higman’s lemma. “The Girard-Reynolds isomorphism,” by Wadler, studies the relationship between the second-order intuitionistic predicate logic, P2, on untyped lambda terms and the second-order lambda calculus, F2; the central result is that the projection from P2 to F2 followed by the embedding from F2 to P2 is the identity in the presence of Reynolds’ parametricity property. “Infinite intersection types,” by Bonsangue and Kok, develops a domain semantics and a type assignment system for the lazy lambda calculus with infinite intersection and union types and proves soundness and completeness of this type assignment system with respect to the semantics. “Non-structural subtype entailment in automata theory,” by Niehren and Priesnitz, studies the open problem of non-structural subtype entailment, shedding new light by applying an automata theoretic approach. The final article is Esparza, Kučera, and Schwoon’s “Model checking LTL with regular valuations for pushdown systems.” It studies model checking for pushdown systems, which is important in