In the dominant view of knowledge bases (KB's), a KB is a set of facts (atomic sentences) and integrity constraints (IC's). An IC is then a sentence which must at least be consistent with the other sentences in the KB. This view obliterates the distinction between, for example, the constraint that age is a natural number (which is true of the universe of discourse (UoD) but may be false in a particular implementation of a KB), and the constraint that a class must have precisely one teacher (which is false of the UoD if a class actually has two teachers). The second constraint is called deontic and constrains the UoD; the first constraint is a necessary truth of the UoD and does not constrain the UoD. Instead, it constrains the implementation of the KB. We argue that the distinction between necessary and deontic IC's is relevant for KB modeling and that it imposes a more complicated modeling discipline on the KB designer than hitherto realized. We show that both types of constraints can be specified in the single framework provided by a deontic variant of dynamic logic, which has the added advantage of being able to specify dynamic constraints as well. We give a simple example to illustrate the difference between dynamic and static specification of deontic IC's, and a non-trivial example of a KB specification with static, dynamic and deontic constraints.

Abstract.An overview is given of work we have done in recent years on the semantics of concurrency, concentrating on semantic models built on metric structures. Three contrasting themes are discussed, viz. (i) uniform or schematic versus nonuniform or interpreted languages; (ii) operational versus denotational semantics, and (iii) linear time versus branching time models. The operational models are based on Plotkin's transition systems. Language constructs which receive particular attention are recursion and merge, synchronization and global nondeterminacy, process creation, and communication with value passing. Various semantic equivalence results are established. Both in the definitions and in the derivation of these equivalences, essential use is made of Banach's theorem for contracting functions.1985 Mathematics Subject Classification: 68Q55, 68Ql0.

Transition systems as proposed by Hennessy and Plotkin are defined for a series of three languages featuring concurrency. The first has shuffie and local nondeterminacy, the second synchronization merge and local nondeterminacy, and the third synchronization merge and global nondeterminacy. The languages are all uniform in the sense that the elementary actions are uninterpreted. Throughout, infinite behaviour is taken into account and modelled with infinitary languages in the sense of Nivat. A comparison with denotational semantics is provided. For the first two languages, a linear time model suffices; for the third language a branching time model with processes in the sense of de Bakker and Zucker is described. In the comparison an important role is played by an intermediate semantics in the style of Hoare and Olderog's specification oriented semantics. A variant on the notion of ready set is employed here. Precise statements are given relating the various semantics terms of a number of abstraction operators. •(t 988 Academic Press, Inc.

In this paper we discuss the role that deontic logic plays in the specification of information systems, either because constraints on the systems directly concern norms or, and even more importantly, system constraints are considered ideal but violable (so-called 'soft' constraints). To overcome the traditional problems with deontic logic (the so-called paradoxes), we first state the importance of distinguishing between ought-to-be and ought-to-do constraints and next focus on the most severe paradox, the so-called Chisholm paradox, involving contrary-to-duty norms. We present a multi-modal extension of standard deontic logic (SDL) to represent the ought-to-be version of the Chisholm set properly. For the ought-to-do variant we employ a reduction to dynamic logic, and show how the Chisholm set can be treated adequately in this setting. Finally we discuss a way of integrating both ought-to-be and ought-to-do reasoning, enabling one to draw conclusions from ought-to-be constraints to ought-to-do ones, and show by an example the use(fulness) of this. 1. there should be no error.

The logic of norms, called deontic logic, has been used to specify normative constraints for information systems. For example, one can specify in deontic logic the constraints that a book borrowed from a library should be returned within three weeks, and that if it is not returned, the library should send a reminder. Thus, the notion of obligation to perform an action arises naturally in system specification. Intuitively, deontic logic presupposes the concept of an actor who undertakes actions and is responsible for fulfilling obligations. However, the concept of an actor has not been formalized until now in deontic logic. We present a formalization in dynamic logic, which allows us to express the actor who initiates actions or choices. This is then combined with a formalization, presented earlier, of deontic logic in dynamic logic, which allows us to specify obligations, permissions, and prohibitions to perform an action. The addition of actors allows us to express who has the responsibility to perform an action. In addition to the application of the concept of an actor in deontic logic, we discuss two other applications of actors. First, we show how to generalize an approach taken up by De Nicola and Hennessy, who eliminate ~ from CCS in favor of internal and external choice. We show that our generalization allows a more accurate specification of system behavior than is possible without it. Second, we show that actors can be used to resolve a long-standing paradox of deontie logic, called the paradox of free-choice permission. Towards the end of the paper, we discuss whether the concept of an actor can be combined with that of an object to formalize the concept of active objects. IntroductionDeontic logic is the logic of permissions, prohibitions, and obligations. Surveys of several deontic logics that have been devised in the past have been given by A1 [20,30] with deontic operators. In earlier papers, we applied Meyer's logic to the specification of conceptual models of information systems [48,51]. We take this application as the point of departure in this paper. The approach is extended with the concept of an actor, and we start with listing some of the reasons why we want to do this. This paper is a revision and an extension of two abstracts that appeared earlier [37,49]. Those abstracts contained a formalization of actors, active choice, that we have replaced, in the current paper, by a formalization in terms of passive and active choices. This allows us to simplify the approach at some points, while at the same time making it more expressive. THE SYSTEM AS ACTOR IN THE UoDIn an earlier paper [48], we specified a library in which an administration of books and library members is maintained. Members can borrow a book for three weeks, and are then obliged to return it. If they do not return it, the library will send a reminder. This is specified in the current version of the logic as Vp, b[borrow(p; b)Formula (1) says that after occurrence of the event borrow(p; b), the obligation predicate O(retur...