Before revision: There are also some fragments of TGDs that can be expressed in guarded logic, e.g., guarded TGDs, where we require that all variables used in the body of a rule must occur together in some atom .
After revision: There are also some fragments of TGDs that can be expressed in guarded logic, in particular: in frontier-guarded TGDs (FGTGD), all the variables shared by the body and the head of a rule (called frontier variables) must occur together in some atom; guarded TGDs (GTGD) are particular FGTGDs where all variables used in the body of a rule must occur together in some atom