  A grounding of a formula $\phi$ over a given finite domain is a ground
formula which is equivalent to $\phi$ on that domain. Very effective
propositional solvers have made grounding-based methods for problem solving
increasingly important, however for realistic problem domains and instances,
the size of groundings is often problematic. A key technique in ground (e.g.,
SAT) solvers is unit propagation, which often significantly reduces ground
formula size even before search begins. We define a "lifted" version of unit
propagation which may be carried out prior to grounding, and describe
integration of the resulting technique into grounding algorithms. We describe
an implementation of the method in a bottom-up grounder, and an experimental
study of its performance.
