  Safe first-order formulas generalize the concept of a safe rule, which plays
an important role in the design of answer set solvers. We show that any safe
sentence is equivalent, in a certain sense, to the result of its grounding --
to the variable-free sentence obtained from it by replacing all quantifiers
with multiple conjunctions and disjunctions. It follows that a safe sentence
and the result of its grounding have the same stable models, and that the
stable models of a safe sentence can be characterized by a formula of a simple
syntactic form.
