Model-finding tools like the Alloy Analyzer produce concrete examples of how a declarative specification can be satisfied. These formal tools are useful in a wide range of domains: software design, security, networking, and more. By producing concrete examples, they assist in exploring system behavior and can help find surprising faults.

Specifications usually have many potential candidate solutions, and yet model- finders tend to leave the choice of which examples to present entirely to the underlying solver. This paper closes that gap by exploring notions of coverage for the model-finding domain, yielding a novel, rigorous metric for output quality. These ideas are realized in the tool CompoSAT, which interposes itself between Alloy's constraint-solving and presentation stages to produce ensembles of examples that maximize coverage.

We show that high-coverage ensembles like those CompoSAT produces are useful for, among other things, detecting overconstraint---a particularly insidious form of specification error. We detail the underlying theory of CompoSAT, discuss its implementation, and evaluate it on numerous specifications.