Ordered Completion for Non-Locally Tight mini-gringo Programs

Abstract: Completion is a well-known transformation that captures the stable model semantics of logic programs by turning a program into a set of first-order definitions. Stable models are models of the completion, but not all models of the completion are stable models. For tight programs (programs without positive recursion) the two semantics coincide. Recently this correspondence was extended to locally tight programs, which avoid non-terminating recursion. However, unlike tightness, local tightness cannot be checked with simple syntactic methods. Completion is crucial for verifying answer set programs, especially for external equivalence: a form of equivalence based on selected output predicates under certain inputs. Standard equivalence and adherence to a first-order specification are special cases of external equivalence. The anthem verification tool has two limitations for checking external equivalence: (1) there is no way to check local tightness automatically, and (2) it is not possible to verify programs that are not locally tight. Therefore, alternatives to completion are of interest. This thesis investigates ordered completion, introduced in [Asuncion et al., 2012], which captures stable models of arbitrary logic programs, but only for finite models. This work extends ordered completion to the mini-gringo language (a subset of the language used by the clingo solver). Additionally, it introduces a modification of ordered completion to handle infinite stable models. This extended ordered completion is implemented in anthem as a translation, and initial experiments demonstrate its use for verifying simple logic programs.