A History of Until

  • Authors:
  • Andrea Masini;Luca Viganò;Marco Volpe

  • Affiliations:
  • Department of Computer Science, University of Verona, Italy;Department of Computer Science, University of Verona, Italy;Department of Computer Science, University of Verona, Italy

  • Venue:
  • Electronic Notes in Theoretical Computer Science (ENTCS)
  • Year:
  • 2010

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Abstract

Until is a notoriously difficult temporal operator as it is both existential and universal at the same time: AUB holds at the current time instant w iff either B holds at w or there exists a time instant w^' in the future at which B holds and such that A holds in all the time instants between the current one and w^'. This ''ambivalent'' nature poses a significant challenge when attempting to give deduction rules for until. In this paper, in contrast, we make explicit this duality of until by introducing a new temporal operator @? that allows us to formalize the ''history'' of until, i.e., the ''internal'' universal quantification over the time instants between the current one and w^'. This approach provides the basis for formalizing deduction systems for temporal logics endowed with the until operator. For concreteness, we give here a labeled natural deduction system for a linear-time logic endowed with the new history operator and show that, via a proper translation, such a system is also sound and complete with respect to the linear temporal logic LTL with until.