Shininess, strong politeness, and unicorns

Shininess and strong politeness are properties related to theory combination procedures. In a paper titled “Many-sorted equivalence of shiny and strongly polite theories”, Casal and Rasga proved that for decidable theories, these properties are equivalent. We refine their result by showing that: (i) shiny theories are always decidable, and therefore strongly polite; and (ii) there are (undecidable) strongly polite theories that are not shiny. This line of research is tightly related to a recent series of papers that have sought to classify all the relations between theory combination properties. We finally complete this project, resolving all of the remaining problems that were previously left open.