Combining Combination Properties, Part I: Nelson-Oppen and Politeness

This is the first part of an analysis of the interplay between multiple properties that are related to combination methodologies for theories in the field of satisfiability modulo theories. We here focus on Nelson-Oppen and polite theory combinations, leading to a total of five model-theoretic properties to be considered: stable infiniteness, smoothness, finite witnessability, strong finite witnessability, and convexity. Our first result is an improvement on polite theory combination, showing that it is possible when only assuming stable infiniteness and strong finite witnessability, and thus implying smoothness is not a prerequisite for this method. Second, we provide examples of Boolean combinations of the aforementioned 5 properties whenever they are possible (e.g., a theory that admits all the properties, a theory that admits none, etc.), sharp in the sense that no theories within simpler signatures may exhibit the exact same properties, and prove which combinations cannot occur. Among these examples, the most surprising one is that of a polite yet not strongly polite theory in one sort, a combination whose previous example in the literature was two-sorted.