Shielded FOND: Planning with Safety Constraints in Pure-Past Linear Temporal Logic

Luigi Bonassi, Giuseppe De Giacomo, Alfonso Emilio Gerevini, Enrico Scala

Abstract: In this paper, we introduce Shielded FOND planning (S-FOND), which is the problem of computing a strategy to reach a final-state goal while preserving a safety specification called shield. In particular, we characterize shields as Pure-Past Linear Temporal Logic formulas that must hold in every prefix of a state trace induced by a solution strategy, thus capturing the whole safety fragment of Linear Temporal Logic formulas over finite traces. We propose three solution encodings for handling S-FOND problems: the first, which is our baseline, simply views a shield as a temporally extended goal; the second, instead, blocks the execution of further actions when the shield gets violated, and the third prevents the execution of actions that could violate the shield by using the notion of regression. We formally prove the correctness of each encoding and experimentally prove their effectiveness over a set of benchmark shields.