Fast and Tiny Structural Self-Indexes for XML

XML document markup is highly repetitive and therefore well compressible using dictionary-based methods such as DAGs or grammars. In the context of selectivity estimation, grammar-compressed trees were used before as synopsis for structural XPath queries. Here a fully-fledged index over such grammars is presented. The index allows to execute arbitrary tree algorithms with a slow-down that is comparable to the space improvement. More interestingly, certain algorithms execute much faster over the index (because no decompression occurs). E.g., for structural XPath count queries, evaluating over the index is faster than previous XPath implementations, often by two orders of magnitude. The index also allows to serialize XML results (including texts) faster than previous systems, by a factor of ca. 2-3. This is due to efficient copy handling of grammar repetitions, and because materialization is totally avoided. In order to compare with twig join implementations, we implemented a materializer which writes out pre-order numbers of result nodes, and show its competitiveness.Comment: 13 page

Political myth and the need for significance :finding ontological security during times of terror

PhD ThesisThis thesis offers a novel theoretical framework for analysing how political and media elites invoke political myths following terror attacks. It does not define political myths as necessarily false claim or untrue stories, but instead draws on the existentialist approaches of Hans Blumenberg and Chiara Bottici to argue that they are form of dramatic narrative that answers human needs for significance (Bedeutsamkeit). Human beings require significance to live in a world that is otherwise indifferent to them or, as Martin Heidegger put it, they are “thrown” into. The thesis thereby connects modern literature on political myth to concept of Angst, most prominently discussed by Søren Kierkegaard and expanded upon by later existentialist philosophers. The thesis elaborates on this with the novel insight that the process of finding significance is also an act of constructing ontological security, and that this is particularly apparent in times of crisis. Following the works of Anthony Giddens and Stuart Croft, the thesis defines ontological security as a condition in which people have constructed a sense of biographical continuity, have a strong web of trust-relations, and are able to avoid Angst. The thesis argues that terror attacks are moments where ontological security (not just physical security) is under threat, and that the process of finding significance (Bedeutsamkeit) through the work on myth simultaneously (re)establishes ontological security. It focuses on two empirical examples: the 7th July 2005 bombings in London and the 2013 Murder of Fusilier Lee Rigby. Following these terror attacks, senior political figures and media commentators invoked a political myth which portrayed the United Kingdom as embroiled in an existential conflict with violent radical Muslims inspired by a warped interpretation of Islam. The thesis concludes that its novel theoretical framework can enable an understanding of discursive responses to other terror attacks across the globe

Path-Specific Objectives for Safer Agent Incentives

We present a general framework for training safe agents whose naive incentives are unsafe. As an example, manipulative or deceptive behaviour can improve rewards but should be avoided. Most approaches fail here: agents maximize expected return by any means necessary. We formally describe settings with 'delicate' parts of the state which should not be used as a means to an end. We then train agents to maximize the causal effect of actions on the expected return which is not mediated by the delicate parts of state, using Causal Influence Diagram analysis. The resulting agents have no incentive to control the delicate state. We further show how our framework unifies and generalizes existing proposals.Comment: Presented at AAAI 202