  Recent methods have adapted the well-established AGM and belief base
frameworks for belief change to cover belief revision in logic programs. In
this study here, we present two new sets of belief change operators for logic
programs. They focus on preserving the explicit relationships expressed in the
rules of a program, a feature that is missing in purely semantic approaches
that consider programs only in their entirety. In particular, operators of the
latter class fail to satisfy preservation and support, two important properties
for belief change in logic programs required to ensure intuitive results.
  We address this shortcoming of existing approaches by introducing partial
meet and ensconcement constructions for logic program belief change, which
allow us to define syntax-preserving operators that satisfy preservation and
support. Our work is novel in that our constructions not only preserve more
information from a logic program during a change operation than existing ones,
but they also facilitate natural definitions of contraction operators, the
first in the field to the best of our knowledge.
  In order to evaluate the rationality of our operators, we translate the
revision and contraction postulates from the AGM and belief base frameworks to
the logic programming setting. We show that our operators fully comply with the
belief base framework and formally state the interdefinability between our
operators. We further propose an algorithm that is based on modularising a
logic program to reduce partial meet and ensconcement revisions or contractions
to performing the operation only on the relevant modules of that program.
Finally, we compare our approach to two state-of-the-art logic program revision
methods and demonstrate that our operators address the shortcomings of one and
generalise the other method.
