Two results on process logic

The paper proposes an extension RPL of the process logic PL of Harel, Kozen and Parikh (1980). The PL formula operators f and suf are replaced by the operators chop and slice, corresponding to Kleene's regular operators · and ∗, thus enabling formulas to express regular sets of paths. The main result is that, in expressive power, PL<RPL, the hard part being in showing that PL⩽RPL. It is argued that this version of PL comes closer to the desired goal of a natural and powerful (yet decidable) logic for reasoning about the ongoing behavior of programs.

Process logic (PL) is a language for reasoning about the behavior of a program during a computation, while propositional dynamic logic (PDL) can only reason about the input-output states of a program. Nevertheless, it is shown that to each PL model M, there corresponds in a natural way a PDL model M^t such that each path in M is represented by a state in M^t. Moreover, to every PL formula p, there corresponds a PDL formula p^t, whose length is linear in that of p, such that p is true of a path in M iff p^t is true of the state which represents that path in M^t. Then it is shown that p is satisfiable iff p^t is satisfiable in a finite PDL model with special properties which is called a pseudomodel. The size of the pseudomodel is in general nonelementary but is bounded by both the depth of nesting of the suf operator and the alternation of the suf and diamond operators. However, for DPL, a deterministic version of PL, the pseudomodel has exponential size, giving a deterministic exponential time procedure for deciding DPL validity. These results suggest that it is the interaction between nondeterministic programs and the suf operator that makes the general decision problem for PL so difficult.

A process logic (PL) is defined that subsumes Pratt's process logic, Parikh's SOAPL, Nishimura's process logic, and Pnueli's Temporal Logic in expressiveness. The language of PL is an extension of the language of Propositional Dynamic Logic (PDL). A deductive system for PL is given which includes the Segerberg axioms for PDL and it is proved that it is complete. It is also shown that PL is decidable.