A New Decidable Class of Tuple Generating Dependencies: The
  Triangularly-Guarded Class
  In this paper we introduce a new class of tuple-generating dependencies
(TGDs) called triangularly-guarded TGDs, which are TGDs with certain
restrictions on the atomic derivation track embedded in the underlying rule
set. We show that conjunctive query answering under this new class of TGDs is
decidable. We further show that this new class strictly contains some other
decidable classes such as weak-acyclic, guarded, sticky and shy, which, to the
best of our knowledge, provides a unified representation of all these
aforementioned classes.
