  Integrating diverse formalisms into modular knowledge representation systems
offers increased expressivity, modeling convenience and computational benefits.
We introduce concepts of abstract modules and abstract modular systems to study
general principles behind the design and analysis of model-finding programs, or
solvers, for integrated heterogeneous multi-logic systems. We show how abstract
modules and abstract modular systems give rise to transition systems, which are
a natural and convenient representation of solvers pioneered by the SAT
community. We illustrate our approach by showing how it applies to answer set
programming and propositional logic, and to multi-logic systems based on these
two formalisms.
