  The Guarded Negation Fragment (GNFO) is a fragment of first-order logic that
contains all positive existential formulas, can express the first-order
translations of basic modal logic and of many description logics, along with
many sentences that arise in databases. It has been shown that the syntax of
GNFO is restrictive enough so that computational problems such as validity and
satisfiability are still decidable. This suggests that, in spite of its
expressive power, GNFO formulas are amenable to novel optimizations. In this
paper we study the model theory of GNFO formulas. Our results include effective
preservation theorems for GNFO, effective Craig Interpolation and Beth
Definability results, and the ability to express the certain answers of queries
with respect to a large class of GNFO sentences within very restricted logics.
  This version of the paper contains streamlined and corrected versions of
results concerning entailment of a conjunctive query from a set of ground facts
and a theory consisting of GNFO sentences of a special form ("dependencies").
