Controlling Reversibility in Reversing Petri Nets with Application to Wireless Communications

Petri nets are a formalism for modelling and reasoning about the behaviour of distributed systems. Recently, a reversible approach to Petri nets, Reversing Petri Nets (RPN), has been proposed, allowing transitions to be reversed spontaneously in or out of causal order. In this work we propose an approach for controlling the reversal of actions of an RPN, by associating transitions with conditions whose satisfaction/violation allows the execution of transitions in the forward/reversed direction, respectively. We illustrate the framework with a model of a novel, distributed algorithm for antenna selection in distributed antenna arrays.Comment: RC 201

Retractable Contracts

In calculi for modelling communication protocols, internal and external choices play dual roles. Two external choices can be viewed naturally as dual too, as they represent an agreement between the communicating parties. If the interaction fails, the past agreements are good candidates as points where to roll back, in order to take a different agreement. We propose a variant of contracts with synchronous rollbacks to agreement points in case of deadlock. The new calculus is equipped with a compliance relation which is shown to be decidable.Comment: In Proceedings PLACES 2015, arXiv:1602.0325

Reversibility in session-based concurrency: A fresh look

Much research has studied foundations for correct and reliable communication-centric software systems. A salient approach to correctness uses verification based on session types to enforce structured communications; a recent approach to reliability uses reversible actions as a way of reacting to unanticipated events or failures. In this paper, we develop a simple observation: the semantic machinery required to define asynchronous (queue-based), monitored communications can also support reversible protocols. We propose a framework of session communication in which monitors support reversibility of (untyped) processes. Main novelty in our approach are session types with present and past, which allow us to streamline the semantics of reversible actions. We prove that reversibility in our framework is causally consistent, and define ways of using monitors to control reversible actions. Keyword

Long-acting β-adrenoceptor agonists in the management of COPD: focus on indacaterol

Bronchodilators are the cornerstone of severe chronic obstructive pulmonary disease (COPD) treatment to improve airflow, symptoms, exercise tolerance, and exacerbations. There is convincing evidence that regular treatment with long-acting bronchodilators is more effective and convenient than treatment with short-acting bronchodilators. Long-acting β-2-agonists include the twice-daily drugs formoterol and salmeterol and, more recently, once-daily indacaterol. Studies with head-to-head comparisons of long-acting bronchodilators are scant, but novel data from controlled trials with the once-daily β(2)-agonist indacaterol indicate superior bronchodilation and clinical efficacy of indacaterol at recommended doses over twice-daily long-acting β(2)-agonists, and at least equipotent bronchodilation compared with once-daily tiotropium. The recent therapeutic developments in COPD underscore a shift from short-acting bronchodilators with multiple dosings per day to reduced dosing frequency and prolonged duration of action, including once-daily treatment, with more consistent effects on various clinical outcomes. This review summarizes relevant clinical data for twice-daily β-2-agonists in COPD, and further focuses on novel data for once-daily indacaterol, including head-to-head comparison trials

The development of the child's understanding of fullness as a ratio.

One of the central problems in the understanding of the child's cognitive development is his acquisition and subsequent modification of concepts. The present study is concerned with the mathematical concept of ratio, and more specifically with the development of the child's understanding of fullness as a ratio. The idea of ratio might be said to involve a quantitative relation between two or more features that remains invariant regardless of the nature of the component features. Such a quantitative relation is typified by the idea of a physical proportion, and it is this which will be investigated: with proportion as embodied in the question, "Which of two containers is fuller (or emptier)?" "Fullness" is a very interesting concept to investigate, for it involves in its very definition a ratio or proportion between the volume of a container and the volume of the substance contained. Thus in the case of proportion, looked at formally, there is an especially great difficulty for the child: in order to make a correct judgment of fullness or emptiness, the child must resist the temptation to follow perceptual appearances, and instead must use a symbolic operation somewhat like computing a ratio. He must estimate the volume of the container that is filled and then relate this volume to the total volume of the container. This is quite_a difficult task for the child to master, but it is plain that in some primitive form the child can deal with 'full' and 'empty' long before he understands ratios. The child's first use of the terms is limited and he does not grasp what is meant by the comparative forms 'fuller' and 'emptier'. However, at some stage the child goes beyond this partial and restricted idea to a more complete understanding of fullness as a ratio. At the Harvard Centre for Cognitive Studies, Jerome S. Bruner (1964, 1966) has recently been concerned with this very aspect of the child's cognitive development. He investigated the concept of fullness by requiring subjects aged five, six, seven, nine and eleven to judge which of two glasses was fuller and which emptier and presented eleven pairs of glasses in all differing with respect to the height of the glasses, the diameter of the glasses, the height of the water, the volume of the water, the volume of unfilled space in each glass, and finally the proportion full and proportion empty. On the basis of his results he claims that it is not until age eleven that the child can resist perceptual cues and judge fullness correctly. There were, however, several shortcomings in both Bruner's design and his procedure which cast doubt upon these findings. It was therefore decided to replicate this experiment modifying and elaborating where considered necessary. (Hanlon, 1967) The results of this experiment showed that the eleven year old child, contrary to Bruner's conclusions, did not fully understand the concept of ratio through the idea of fullness so the age range studied was subsequently extended to 15 years (Moore and McWhirter, in press). However, it soon became clear that even the older children were not responding correctly, and it seemed that the concept was causing difficulty through misunderstanding rather than inability to perform the necessary operations. The children judged according to their interpretation of the concept and in so doing gave sporadic glimpses that if the problem were communicated to them more precisely they could respond correctly. A further extension was therefore undertaken in which an attempt was made to communicate the problem more exactly to a representative sample of children in the same age range and a comparison was also made between the performance of adult subjects in the original task and in the new experiment where aid was given. The original subjects served - 3 - as a control group. The results were considered both within the context of the development of the child's understanding of fullness as a ratio and also in relation to the findings of Bruner and Kenney