Bidirectional Transformation "bx" (Dagstuhl Seminar 11031)

Bidirectional transformations bx are a mechanism for maintaining the consistency of two (or more) related sources of information. Researchers from many different areas of computer science including databases (DB), graph transformations (GT), software engineering (SE), and programming languages (PL) are actively investigating the use of bx to solve a diverse set of problems. Although researchers have been actively working on bidirectional transformations in the above mentioned communities for many years already, there has been very little cross-discipline interaction and cooperation so far. The purpose of a first International Meeting on Bidirectional Transformations (GRACE-BX), held in December 2008 near Tokyo, was therefore to bring together international elites, promising young researchers, and leading practitioners to share problems, discuss solutions, and open a dialogue towards understanding the common underpinnings of bx in all these areas. While the GRACE-BX meeting provided a starting point for exchanging ideas in different communities and confirmed our believe that there is a considerable overlap of studied problems and developed solutions in the identified communities, the Dagstuhl Seminar 11031 on ``Bidirectional Transformations\u27\u27 also aimed at providing a place for working together to define a common vocabulary of terms and desirable properties of bidirectional transformations, develop a suite of benchmarks, solve some challenging problems, and launch joint efforts to form a living bx community of cooperating experts across the identified subdisciplines. This report documents the program and the outcomes of Dagstuhl Seminar 11031 with abstracts of tutorials, working groups, and presentations on specific research topics

Using automata to characterise fixed point temporal logics

This work examines propositional fixed point temporal and modal logics called mu-calculi and their relationship to automata on infinite strings and trees. We use correspondences between formulae and automata to explore definability in mu-calculi and their fragments, to provide normal forms for formulae, and to prove completeness of axiomatisations. The study of such methods for describing infinitary languages is of fundamental importance to the areas of computer science dealing with non-terminating computations, in particular to the specification and verification of concurrent and reactive systems. To emphasise the close relationship between formulae of mu-calculi and alternating automata, we introduce a new first recurrence acceptance condition for automata, checking intuitively whether the first infinitely often occurring state in a run is accepting. Alternating first recurrence automata can be identified with mu-calculus formulae, and ordinary, non-alternating first recurrence automata with formulae in a particular normal form, the strongly aconjunctive form. Automata with more traditional Büchi and Rabin acceptance conditions can be easily unwound to first recurrence automata, i.e. to mu-calculus formulae. In the other direction, we describe a powerset operation for automata that corresponds to fixpoints, allowing us to translate formulae inductively to ordinary Büchi and Rabin-automata. These translations give easy proofs of the facts that Rabin-automata, the full mu-calculus, its strongly aconjunctive fragment and the monadic second-order calculus of n successors SnS are all equiexpressive, that Büchi-automata, the fixpoint alternation class Pi_2 and the strongly aconjunctive fragment of Pi_2 are similarly related, and that the weak SnS and the fixpoint-alternation-free fragment of mu-calculus also coincide. As corollaries we obtain Rabin's complementation lemma and the powerful decidability result of SnS. We then describe a direct tableau decision method for modal and linear-time mu-calculi, based on the notion of definition trees. The tableaux can be interpreted as first recurrence automata, so the construction can also be viewed as a transformation to the strongly aconjunctive normal form. Finally, we present solutions to two open axiomatisation problems, for the linear-time mu-calculus and its extension with path quantifiers. Both completeness proofs are based on transforming formulae to normal forms inspired by automata. In extending the completeness result of the linear-time mu-calculus to the version with path quantifiers, the essential problem is capturing the limit closure property of paths in an axiomatisation. To this purpose, we introduce a new \exists\nu-induction inference rule

Symbolic execution: a semantic approach

AbstractThis paper discusses symbolic execution from a semantic point of view, covering both programs and specifications. It defines the denotational semantics of symbolic execution of specifications and programs, and thus introduces a notion of correctness of symbolic execution which applies not just to an individual language but to a wide class of languages, namely those whose semantics can be described in terms of states and state transformations. Also described are the operational semantics of a language as used for symbolic execution.This work also provided the basis of the prototype symbolic execution system SYMBEX which was developed at the University of Manchester as part of the mural project. However, this paper only covers the theoretical foundations used by SYMBEX, but not the system itself

Beijing Underground

This thesis investigates a unique type of migrant housing in Beijing: underground hostels retrofitted from civil air defence basements. The core of this study consists of field research conducted from 2005-2006. Personal narratives, photographs, maps, and illustrations drawn from first-person experience construct an account of a neglected layer of the city. Political and economic reforms since the late 1970s initiated the formation of a new subaltern class in contemporary Chinese cities known as the floating population. Millions of migrants have flowed through China’s uneven economic landscape in pursuit of the Chinese Dream. There is an estimated 4 million migrants actively contributing to the construction of new Beijing, yet the subalterns are excluded from official State representations that focus on the monumental. Without proper household registration (hukou) status, rural migrants have little to no access to social welfare including subsidized housing. Migrants have, of necessity, developed unconventional habitats in the capital city. In the absence of officially sanctioned space, migrants seek shelter within cracks and fissures of the formal system. Just as the city is being shaped by the flow of capital, the inflow of the floating population shapes an alternative urban geography that remains largely invisible. Sanctioned yet unofficial, the migrants’ creative appropriation of space contributes to the development of an emergent urban vernacular. Portraits from below reveal furtive portions of Beijing: marginal, banal, and hidden stages upon which life unfolds