Variant-Based Decidable Satisfiability in Initial Algebras with Predicates

Decision procedures can be either theory-specific, e.g., Presburger arithmetic, or theory-generic, applying to an infinite number of user-definable theories. Variant satisfiability is a theory-generic procedure for quantifier-free satisfiability in the initial algebra of an order-sorted equational theory $(Σ,E \cup B)$ under two conditions: (i) $E \cup B$ has the finite variant property and $B$ has a finitary unification algorithm; and (ii) $(Σ,E \cup B)$ protects a constructor subtheory $(Ω,E_Ω \cup B_Ω)$ that is OS-compact. These conditions apply to many user-definable theories, but have a main limitation: they apply well to data structures, but often do not hold for user-definable predicates on such data structures. We present a theory-generic satisfiability decision procedure, and a prototype implementation, extending variant-based satisfiability to initial algebras with user-definable predicates under fairly general conditions.