The program psmodels is a modification of smodels that can be used to compute preferred answer sets under the ordered disjunction semantics. Psmodels also needs lparse version 1.0.10 or later to generate the input files.
An ordered disjunctions is of the form
a x b :- body.
and informally it means that if body is true, then
a should be true. If a can't be true,
then b has to be true. The formal definition can
be found in:
The psmodels distribution archive contains a README file that explains how psmodels and lparse work in common.