  We are aiming at a semantics of logic programs with preferences defined on
rules, which always selects a preferred answer set, if there is a non-empty set
of (standard) answer sets of the given program. It is shown in a seminal paper
by Brewka and Eiter that the goal mentioned above is incompatible with their
second principle and it is not satisfied in their semantics of prioritized
logic programs. Similarly, also according to other established semantics, based
on a prescriptive approach, there are programs with standard answer sets, but
without preferred answer sets. According to the standard prescriptive approach
no rule can be fired before a more preferred rule, unless the more preferred
rule is blocked. This is a rather imperative approach, in its spirit.
  In our approach, rules can be blocked by more preferred rules, but the rules
which are not blocked are handled in a more declarative style, their execution
does not depend on the given preference relation on the rules. An argumentation
framework (different from the Dung's framework) is proposed in this paper.
Argu- mentation structures are derived from the rules of a given program. An
attack relation on argumentation structures is defined, which is derived from
attacks of more preferred rules against the less preferred rules. Preferred
answer sets correspond to complete argumentation structures, which are not
blocked by other complete argumentation structures.
