  U-Datalog has been developed with the aim of providing a set-oriented logical
update language, guaranteeing update parallelism in the context of a
Datalog-like language. In U-Datalog, updates are expressed by introducing
constraints (+p(X), to denote insertion, and [minus sign]p(X), to denote
deletion) inside Datalog rules. A U-Datalog program can be interpreted as a CLP
program. In this framework, a set of updates (constraints) is satisfiable if it
does not represent an inconsistent theory, that is, it does not require the
insertion and the deletion of the same fact. This approach resembles a very
simple form of negation. However, on the other hand, U-Datalog does not provide
any mechanism to explicitly deal with negative information, resulting in a
language with limited expressive power. In this paper, we provide a semantics,
based on stratification, handling the use of negated atoms in U-Datalog
programs, and we show which problems arise in defining a compositional
semantics.
