Hybrid Knowledge Bases for Real-Time Robotic Reasoning

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Final Report June 1992-June 1995
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Computation and Implementation of Non-Monotonic
Deductive Databases
6. AUTHOR(S)
V.S. Subrahmanian
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University of >laryland
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13. ABSTRACT (Maximum 200 words)
This is the final report on a 3-year grant from the Army Research Office to the
university of Maryland to conduct research in the area of "Computation and Implem-
entation of NonTMonotonic Deductive Databases.''' The main goal of the research was
to study methods of efficient, scalable reasoning about situations where only in-
complete and/or uncertain information is available. Towards this end, we conducted
research on databases that contain incomplete symbolic information-such databases
are typically called ^'nonmonotonic deductive databases." We also studied systems
that contain incomplete information> but where different probability measures are
available specifying partial information about the missing information.
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Final Project Report
July 15, 1995
Computation and Implementation
of Nonmonotonic Deductive Databases
ARO Grant No. DAAL-03-92-G-0225
Principal Investigator: V.S.Subrahmanian
Department of Computer Science
University of Maryland
College Park, MD 20742.
This is the final report on a 3-year grant from the Army Research Office to the
University of Maryland to conduct research in the area of "Computation and Imple-
mentation of Nonmonotonic Deductive Databases." The main goal of the research
was to study methods of efficient, scalable reasoning about situations where only in-
complete and/or uncertain information is available. Towards this end, we conducted
research on databases that contain incomplete symbolic information - such databases
are typically called nonmonotonic deductive databases. We also studied systems that
contain incomplete information, but where different probability measures are available
specifying partial information about the missing information.
We are pleased to report that this grant has led to 49 publications, including
ones in prestigious archival journals such as The Journal of the ACM, ACM Transac-
tions on Database Systems, IEEE Transactions on Knowledge and Data Engineering,
Information and Computation, and many others.
1 Summary of Accomplishments
The research performed under this grant has been on a variety of topics related to the
computation and implementation of nonmonotonic deductive databases and reasoning
with heterogeneous databases, data structures and software. The research performed
under this grant during this period may be classified into the following categories:
1. Computing Minimal Models by Integer Programming. Existing and
past generations of Prolog compilers have left deduction to run-time and this
may account for the poor run-time performance of existing Prolog systems. Our
work tries to minimize run-time deduction by shifting the deductive process
to compile-time, not run-time. Additionally, we offer an alternative inferenc-
ing procedure based on translating logic to mixed integer programming. This
makes available for research and implementation in deductive databases, all the
theorems, algorithms, and software packages developed by the operations re-
search community over the past fifty years. The method keeps the same query
language as for disjunctive deductive databases, only the inferencing procedure

changes. The language is purely declarative, independent of the order of rules in
the program, independent of the order in which literals occur in clause bodies.
The technique avoids Prolog's problem of infinite looping. It saves run time by
doing primary inferencing at compile time. Furthermore, it is incremental in
nature.
2. Computing Stable Models by Integer Programming. Though the declar-
ative semantics of both explicit and nonmonotonic negation in logic programs
has been studied extensively, relatively little work has been done on computation
and implementation of these semantics. In this work, we study three different
approaches to computing stable models of logic programs based on mixed in-
teger linear programming methods for automated deduction introduced by R.
Jeroslow. We subsequently study he relative efficiency of these algorithms. The
results of experiments with a prototype compiler implemented by us tend to
confirm our theoretical discussion. In contrast to resolution, the mixed integer
programming methodology is both fully declarative and handles reuse of old
computations gracefully.
We also introduce, compare, implement, and experiment with linear constraints
corresponding to four semantics for "explicit" negation in logic programs: the
four-valued annotated semantics of Blair and Subrahmanian, the Gelfond-Lifschitz
semantics, the over-determined model semantics of Grant and Subrahmanian
and the classical logic semantics. Gelfond and Lifschitz argue for simultaneous
use of two modes of negation in logic programs, "classical" and "nonmonotonic",
so we give algorithms for computing "answer sets" for such logic programs too.
3. Computing Well-Founded and Stable Models. Though the semantics of
non-monotonic logic programming has been studied extensively, relatively little
work has been done on operational aspects of of these semantics. In this work,
we develop techniques to compute the well-founded model of a logic program.
We describe a prototype implementation and show, based on experimental re-
sults, that our technique is more efficient than the standard alternating fixpoint
computation. Subsequently, we develop techniques to compute the set of all
stable models of a deductive database. These techniques first compute the well-
founded semantics and then use an intelligent branch and bound strategy to
compute the stable models. We report on our implementation, as well as on
experiments that we have conducted on the efficiency of our approach.
4. Computing Circumscriptive Databases. Though circumscription was in- 3F
-*>
troduced by McCarthy over a decade ago, there has been relatively little work on — Jh
algorithms for computing circumscriptive databases. In this work, we develop r-> J|
algorithms to compute the preferred models of circumscriptive databases at Q
compile-time using mixed integer linear programming techniques. Two advan- r
tages of this (bottom-up) approach are that it makes efficient re-use of previous ™~
computations and it provides much faster run-time performance. Some other •—
advantages of using linear programming to automate deduction at compile time —
is that its re-optimization facilities elegantly accommodate database updates >CÖ
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Mat Spöoiai.

and also that it leads to a completely declarative formulation in which ordering
of rules and literals in rule bodies plays no real role. Finally, we plan to use a
standard relational database system as our run-time environment; this should
yield relatively fast run-time processing, and provide a more expressive query
language in which aggregates and the like can be expressed easily.
Partial Instantiation for Definite Programs. Query processing in ground
definite deductive databases is known to correspond precisely to a linear pro-
gramming problem. However, the "groundedness" requirement is a huge draw-
back to using linear programming techniques for logic program computations
because the ground version of a logic program can be very large when compared
to the original logic program. Furthermore, when we move from prepositional
logic programs to first order logic programs, this effectively means that func-
tions symbols may not occur in clauses. In this work, we develop a theory
of "instantiate-by-need" that performs instantiations (not necessarily ground
instantiations) only when needed. We prove that this method is sound and
complete when computing answer substitutions for non-ground logic programs
including those containing function symbols. More importantly, when taken in
conjunction with Colmerauer's result that unification can be viewed as linear
programming, this means that resolution with unification can be completely
replaced by linear programming as an operational paradigm. Additionally, our
tree construction method is not rigidly tied to the linear programming paradigm
- we will show that given any method M (which some implementors may pre-
fer) that can compute the set of atomic logical consequences of a propositional
logic program, our method can use M to compute the set of all (not necessarily
ground) atoms that are consequences of a first-order logic program.
Partial Instantiation for Minimal Model Computations. Unlike sets of
definite Horn clauses, logic programs with disjunctions of atoms in clause heads
are often interpreted in terms of minimal models. It is also well known that
the minimal models of logic programs are closely related to the so-called stable
models of logic programs with non-monotonic negation in clause bodies, as well
as to circumscription. Methods to compute minimal models of logic programs
are becoming increasingly important as an intermediate step in the compu-
tation of structures associated with nonmonotonic logic programs. However,
to date, all these techniques have been restricted to the case of propositional
logic programs which means that an ordinary disjunctive logic program must be
"grounded out" prior to computation. Grounding out in this manner leads to
a combinatorial explosion in the number of clauses, and hence, is unacceptable.
In this work, we show how, given any method M which correctly computes
the set of minimal models of a propositional logic program, we can develop a
strategy to compute truth in a minimal model of a disjunctive logic program P.
The novel feature of our method is that it works on an "instantiate-by-need"
basis, and thus avoids unnecessary grounding.

7. Non-Ground Stable and Well-Founded Semantics for Nonmonotonic
Deductive Databases: The declarative semantics of nonmonotonic logic pro-
gramming has largely been based on propositional programs. However, the
ground instantiation of a logic program may be very large, and likewise, a
"ground" stable model may also be very large. We develop a non-ground se-
mantic theory for nonmonotonic logic programming. The principal advantage
of this is that stable models and well-founded models can be represented as sets
of atoms, rather than as sets of ground atoms. A set S of atoms may be viewed
as a (usually more) compact representation of the set of ground instances of
the atoms in S. We develop generalizations of the stable and well-founded se-
mantics of logic programs based on this non-ground interpretation. The key
technical basis for our theory is that of an "anticover" of a set of substitutions.
Intuitively, an anticover of a set X of substitutions is a set of substitutions, all
of whom are incompatible (i.e. they share no common instance) with the sub-
stitutions in X, and such that each substitution that is incompatible with all
members of X is an instance of some substitution in the anticover. We develop
methods of computing anticovers, show that membership in certain anticovers
called optimal anticovers is decidable, and develop special complexity results in
the Datalog case.
8. Probabilistic Databases. Of all scientific investigations into reasoning with
uncertainty and chance, probability theory is perhaps the best understood
paradigm. Nevertheless, all studies conducted thus far into the semantics of
quantitative logic programming (cf. van Emden, Fitting, Blair and Subrah-
manian, Kifer et al have restricted themselves to non-probabilistic semantical
characterizations. In this work, we take a few steps towards rectifying this sit-
uation. We define a logic programming language that is syntactically similar
to annotated logics, but in which the truth values are interpreted probabilisti-
cally. A probabilistic model theory and fixpoint theory is developed for such
programs. This probabilistic model theory satisfies the requirements proposed
by Fenstad for a function to be called probabilistic. The logical treatment of
probabilities is complicated by two facts: first, that the connectives cannot be
interpreted truth functionally when truth values are regarded as probabilities;
second that negation-free definite clause like sentences can be inconsistent when
interpreted probabilistically. We address these issues here and propose a formal-
ism for probabilistic reasoning in logic programming. To our knowledge, this
is the first probabilistic characterization of logic programming semantics. Our
work is closely related to current work of Fitting, Fagin, Halpern and Megiddo,
van Emden, Kifer et al and Blair and Subrahmanian.
9. Probabilities and Nonmonotonic Reasoning: We study the the seman-
tics of non-monotonic negation in probabilistic deductive databases. Based on
the stable semantics for classical logic programming, we examine three natural
notions of stability: stable formula functions, stable families of probabilistic
interpretations and stable probabilistic models. We show that stable formula
functions are minimal fixpoints of operators associated with probabilistic logic

programs. We also prove that each member in a stable family of probabilistic
interpretations is a probabilistic model of the program. Then we show that
stable formula functions and stable families behave as duals of each other, tying
together elegantly the fixpoint and model theories for probabilistic logic pro-
grams with negation. Furthermore, since a probabilistic logic program may not
necessarily have a stable family of probabilistic interpretations, we provide a
stable class semantics for such programs. Finally, we investigate the notion of
stable probabilistic model. We show that this notion, though natural, is too
weak in the probabilistic framework.
10. Efficient View Maintenance. We present incremental evaluation algorithms
to compute changes to materialized views in relational and deductive database
systems, in response to changes (insertions, deletions, and updates) to the re-
lations. The view definitions can be in SQL or Datalog, and may use UNION,
negation, aggregation (e.g. SUM, MIN), linear recursion, and general recursion.
We first present a counting algorithm that tracks the number of alternative
derivations (counts) for each derived tuple in a view. The algorithm works with
both set and duplicate semantics. We present the algorithm for nonrecursive
views (with negation and aggregation), and show that the count for a tuple
can be computed at little or no cost above the cost of deriving the tuple. The
algorithm is optimal in that it computes exactly those view tuples that are
inserted or deleted. Note that we store only the number of derivations, not the
derivations themselves.
We then present the Delete and Rederive algorithm, DRED, for incremental
maintenance of recursive views (negation and aggregation are permitted). The
algorithm works by first deleting a superset of the tuples that need to be deleted,
and then rederiving some of them. The algorithm can also be used when the
view definition is itself altered.
11. Hybrid Knowledge Bases Deductive databases that interact with, and are
accessed by, reasoning agents in the real world (such as logic controllers in
automated manufacturing, weapons guidance systems, aircraft landing systems,
land-vehicle maneuvering systems, and air-traffic control systems) must have
the ability to deal with multiple modes of reasoning. Specifically, the types
of reasoning we are concerned with include, amongst others, reasoning about
time, reasoning about quantitative relationships that may be expressed in the
form of differential equations or optimization problems, and reasoning about
numeric modes of uncertainty about the domain which the database seeks to
describe. Such databases may need to handle diverse forms of data structures,
and frequently they may require use of the assumption-based non-monotonic
representation of knowledge.
A hybrid knowledge base is a theoretical framework capturing all the above
modes of reasoning. The theory tightly unifies the Constraint Logic Program-
ming Scheme of Jaffar and Lassez, the Generalized Annotated Logic Program-
ming Theory of Kifer and Subrahmanian, and the Stable Model semantics of

Gelfond and Lifschitz. New techniques are introduced which extend both the
work on Annotated Logic Programming and the Stable Model semantics.
12. Integrating Relational Databases: Previous research on integrating mul-
tiple relational databases do not consider the problem of what to do when
integrity constraints are violated by the amalgamation of relational databases,
even though the individual databases participating in the amalgamation satisfy
the integrity constraints. We propose three new data retrieval notions based
on whether the constraint semantics is "naive", "skeptical" or makes "choices."
We propose a semantics for these operations, and develop an algebra and cal-
culus based on these operators. We prove that the algebra can be embedded
within the calculus - however, the calculus is strictly more powerful than the
algebra. We study various algebraic properties linking the newly defined oper-
ators together and show how these algebraic properties can be used for query
optimization.
13. Distributed Knowledge Integration: Integrating knowledge from multiple
sources is an important aspect of automated reasoning systems. In previous
work, we presented a uniform declarative and operational framework, based on
annotated logics, for amalgamating multiple knowledge bases and data struc-
tures (e.g. relational, object-oriented, spatial, and temporal structures) when
these knowledge bases (possibly) contain inconsistencies, uncertainties, and non-
monotonic modes of negation. We showed that annotated logics may be used,
with some modifications, to mediate between different knowledge bases. The
multiple knowledge bases are amalgamated by embedding the individual knowl-
edge bases into a lattice. In this work, we describe how, given a network of sites
where the different databases reside, it is possible to define a distributed se-
mantics for amalgamated knowledge bases. More importantly, we study how
the mediator may be distributed across multiple sites so that when certain con-
ditions are satisfied, network failures do not affect the end results of queries
that a user may pose. We specify different ways of distributing the mediator to
protect against different types of network link failures and develop alternative
soundness and completeness results.
14. Efficient Querying of Amalgamated Knowledge Bases: Integrating knowl-
edge from multiple sources is an important aspect of automated reasoning sys-
tems. In this work, we briefly describe an SLD-resolution based proof procedure
that is sound and complete w.r.t. our declarative semantics. We will then de-
velop an OLDT-resolution based query processing procedure, MULTI-OLDT, that
satisfies two important properties: (1) efficient reuse of previous computations
is achieved by maintaining a table - we describe the structure of this table and
show that table operations can be efficiently executed, and (2) approximate,
interruptable query answering is achieved, i.e. it is possible to obtain an "inter-
mediate, approximate" answer from the QPP by interrupting it at any point in
time during its execution. The design of the MULTIJOLDT procedure will include:
(1) development of data structures for tabling (substitution, truth value) pairs,

and (2) the development of algorithms to incrementally and efficiently update
the table.
15. Missile Siting Probelms: Hybrid knowledge bases are a formalism for in-
tegrating multiple representations of knowledge and data. HKBs provide a
uniform framework for integrating uncertain information (as is often the case
in terrain reasoning), temporal information (needed for weather effects, etc),
and numeric constraint solving capabilities (for situation assessment). We show
how the HKB formalism may be applied to solve the problem of placing Patriot
and Hawk missile batteries in a specified terrain subject to the requirement that
various existing assets be afforded maximal protection. We formalize this prob-
lem in a clear, mathematical framework, using the HKB paradigm, and show
how the problem is solved. This provides a mathematically sound, as well as a
practically viable, scalable solution to the important problem of missile siting.
16. Linking Planning and Non-Monotonic Reasoning: Jointly with carlo
Zaniolo, we have shown that there is a simple connection between logic pro-
gramming and planning. The main result of our work is the following: given
any planning domain consisting of an initial state, and a set of operation defi-
nitions, this domain can be translated, in linear-time, to a logic program such
that a given goal G is achievable in the planning domain iff a related goal G*
is true in some stable model of the logic program obtained by the translation.
We show that this translation yields at least two interesting consequences: (1)
methods to update databases can be used to handle surprises when executing
plans (i.e. a surprise occurs when an initial plan is partly executed, but one of
the resulting intermediate states differs, perhaps due to external reasons, from
what is predicted). (2) rigid actions, which are actions that must be executed
when their pre-conditions are true, can be easily accommodated within our
framework as well.
17. Temporal Databases: In a federated database environment, different con-
stituents of the federation may use different temporal models or physical repre-
sentations for the temporal information. This work introduces a new concept,
called a temporal module, to resolve these mismatches among the constituents.
Intuitively, a temporal module hides the implementation details of a temporal
relation by exposing its information only through two windowing functions: a
function associating each time point with a set of tuples and a function linking
each tuple to a set of time points. A calculus-style language is given to form
queries on temporal modules.
Temporal modules are then extended to resolve another type of mismatch among
the constituents of a federation, namely, mismatch involving different time units
(e.g., month, week and day) used in recording temporal information. Our so-
lution to this mismatch relies on "information conversions" provided by each
constituent. Specifically, a temporal module is extended to provide several "win-
dows" to its information, each in terms of a different time unit. The first step to
process a query addressed to the federation is to select suitable windows for the

underlying temporal modules. In order to do so, time units axe formally defined
and studied. A federated temporal database model and its query language are
proposed. The query language is an extension of the calculus-style language
above.
18. Security in Deductive databases: In this work, we develop a formal logical
foundation for secure deductive databases. This logical foundation is based on
an extended logic involving several modal operators. We develop two models
of interaction between the user and the database called "yes-no" dialogs, and
"yes-no-don't know" dialogs. Both dialog frameworks allow the database to lie
to the user. We develop an algorithm for answering queries using yes-no dialogs
and prove that secure query processing using yes-no dialogs is NP-complete.
Consequently, the degree of computational intractability of query processing
with yes-no dialogs is no worse than for ordinary databases. Furthermore, the
algorithm is maximally cooperative to user in the sense that lying is resorted to
only when absolutely necessary. For Horn databases, we show that secure query
processing can be achieved in linear time - hence, this is no more intractable
than the situation in ordinary databases. Finally, we identify necessary and
sufficient conditions for the database to be able to preserve security. Similar
results are also obtained for yes-no-don't know dialogs.
19. Theorem Proving with Equality: We show that the algorithm directly
induced by the viability definition of Digricoli and Harrison does not terminate
in general. As a consequence, RUE-resolution in strong form is not complete.
Moreover, we show that ground query processing for covered pure logic programs
can be reduced to computing viability. Since the problem of ground query
processing is strictly recursively enumerable even under the above restrictions,
it follows that the notion of viability is undecidable. Finally, we present a
modified viability check that solves the non-termination problem for ground
terms.
2 Technology Transfer: Applications to the Army's
Missile Siting Problems.
Jointly with John Benton of the US Army Topographic and Engineering Center,
Corps, of Engineers, Ft. Belvoir, VA, we studied the use of deductive databases as
an efficient way of siting Patriot and Hawk Missile Batteries. In a war-time situation,
Patriot and Hawk missile batteries are used to protect the front line against enemy
attack. If the batteries could simply be positioned and left in place for the duration
of the war, then the task of manually siting these batteries would not be onerous.
Unfortunately, this is normally not the case during a shooting war. On a battlefront,
these weapons systems may be moved several times in one week and there must be
several contingency sites available for each battery. In particular, Hawk batteries have
strict requirements for locating sites with optimum line-of-sight visibility, and even

with current computer-assisted planning systems, siting the batteries is a trial-and-
error based, time-consuming task. Patriot Missile Batteries can be used to defend
against either air-breathing threats such as jet aircraft or cruise missiles or against
ballistic missile threats such as the Scud missile. Deployment patterns for the two
tasks are quite different. In the work to date, only the air breathing case is considered.
We show that the HKB framework framework may be used to naturally represent the
problem of siting Patriot and Hawk missile batteries in order to protect assets located
in a given theater of operations.
Contact with Army Personnel: I are delighted to report a healthy, continuing
ongoing interaction between myself and the AI group at the US Army Topographic
and Engineering Center (TEC). In particular, since the beginning of this grant, I
have visited TEC approximately 30-35 times, and John Benton and Ann Werkheiser
of TEC have visited the University of Maryland at least on 20 occasions. Furthermore,
Anne Brink of TEC has visited us at least 10 times. John Benton and I have been
interacting on the applications of hybrid knowledge base technology to the problem
of siting Patriot and Hawk missile batteries. Hybrid knowledge bases are a class of
knowledge bases used to integrate diverse representations of data, and, in some cases,
diverse reasoning paradigms.
3 Graduate Students
One PhD student (Raymond Ng) graduated while being supported, in part, by this
grant. Another student received a Master's Thesis (Ross Emery). Other students
who have been supported, in part, by this grant include Tom Weaver, Sibel Adali
and K.S. Candan.
4 Publications
1. S. Adali, K.S. Candan, S.-S. Chen, K. Erol, and V.S.Subrahmanian. Advanced
Video Information Systems. Submitted to ACM MULTIMEDIA JOURNAL, cur-
rently being revised according to referee comments.
2. V.S. Subrahmanian. Amalgamating Knowledge Bases, ACM TRANSACTIONS
ON DATABASE SYSTEMS, 19, 2, pp. 291-331, 1994.
3. S. Adali and V.S. Subrahmanian. Amalgamating Knowledge Bases, II: Dis-
tributed Mediators; has appeared in: INTERNATIONAL JOURNAL OF INTELLI-
GENT COOPERATIVE INFORMATION SYSTEMS, vol. 3, No. 4 (1994), 349-383.
4. S. Adali and V.S. Subrahmanian. Amalgamating Knowledge Bases, III: Algo-
rithms, Data Structures and Query Processing; UNIVERISTY OF MARYLAND
TECHNICAL REPORT. Accepted for publication in Journal of Logic Program-
ming.

5. Kasim S. Candan, V.S. Subrahmanian. An Algebra and Calculus for Multi-
databases with Integrity Constraints; UNIVERSITY OF MARYLAND TECHNICAL
REPORT.
6. S. Pradhan, J. Minker and V.S. Subrahmanian. Combining Databases with Pri-
oritized Information, accepted for publication in: JOURNAL OF INTELLIGENT
INFORMATION SYSTEMS.
7. C. Baral, S. Kraus, J. Minker, V.S. Subrahmanian. Combining Default Logic
Databases. To appear in Journal of Intelligent Information Systems, 1995.
8. J. Lu and V.S. Subrahmanian. Completeness Issues in RUE-NRF Deduction,
JOURNAL OF AUTOMATED REASONING, 10, pps 371-388, 1993.
9. J. Minker and V.S. Subrahmanian. Completion Theoretic Semantics for Dis-
junctive Logic Programs Proc. 1990 Int. Symp. on Methodologies for Intelligent
Systems, pps 545-552, Knoxville, Tennessee, North Holland. Aug. 1990.
10. K. Erol, D.S. Nau and V.S. Subrahmanian. Complexity, Decidability and Unde-
cidability Results for Domain-Independent Planning, accepted for publication
in ARTIFICIAL INTELLIGENCE journal, Special Issue on Planning and Schedul-
ing.
11. Anil Nerode, Raymond T. Ng, V.S. Subrahmanian. Computing Circumscriptive
Databases, Part 1: Theory and Algorithms, appeared in INFORMATION AND
COMPUTATION, VOL. 116, NR. 1, PPS 58-80.
12. Vadim Kagan, Anil Nerode, V.S. Subrahmanian. Computing Definite Logic
Programs by Partial Instantiation and Linear Programming, appeared in AN-
NALS OF PURE AND APPLIED LOGIC 67 (1994), PPS 161-182.
13. V. Kagan, A. Nerode and V.S. Subrahmanian. Computing Minimal Models by
Partial Instantiation, accepted for publication in: THEORETICAL COMPUTER
SCIENCE.
14. P. Bonatti, S. Kraus and V.S. Subrahmanian. Declarative Foundations of Secure
Deductive Databases, Proc. 1992 Intl. Conf. on Database Theory, Lecture
Notes in Computer Science, Vol. 646, pp. 391-406, Springer.
15. J. Fernandez, J. Lobo, J. Minker and V.S. Subrahmanian. Disjunctive LP +
Integrity Constraints = Stable Model Semantics, ANNALS OF MATHEMATICS
AND ARTIFICIAL INTELLIGENCE, Vol. 8, pps 449-474, 1993.
16. J. Lu, G. Moerkotte, J. Schue, and V.S. Subrahmanian. Efficient Maintenance
of Materialized Mediated Views, in: Proc. 1995 ACM SIGMOD Conf. on
Management of Data, San Jose, CA, May 1995.
17. J. Benton, S.S. Iyengar, W. Deng, N. Brener, and V.S. Subrahmanian. Fine-
Grained Terrain Data: A New Computational Challenge for Route Planners,
submitted to Artificial Intelligence Journal.
10

18. P. Bonatti, S. Kraus and V.S. Subrahmanian. Foundations of Secure Deductive
Databases, in: IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGI-
NEERING, Vol. 7, no. 3, June 1995.
19. S. Marcus and V.S. Subrahmanian. Foundations of Multimedia Information
Systems, submitted to The Journal of the ACM.
20. V.S. Subrahmanian, S. Adali, A. Brink, R. Emery, J. Lu, A. Rajput, T.J.
Rogers, R. Ross and C. Ward. HERMES: A Heterogeneous Reasoning and
Mediator System. University of Maryland Technical Report.
21. A. Brink, S. Marcus and V.S. Subrahmanian. Heterogeneous Multimedia Rea-
soning. Accepted for publication in: IEEE COMPUTER, SEPTEMBER 1995.
22. J. Lu, A. Nerode and V.S. Subrahmanian. Hybrid Knowledge Bases, accepted
for publication in: IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGI-
NEERING.
23. John Horst, Ernest Kent, Hassan Rifky, V.S. Subrahmanian. (ed. E. Gelsema
and. L. Kanal). Hybrid Knowledge Bases for Real-Time Robotic Reasoning, in
Proc. Pattern Recognition in Practice IV Conference, June 1994, N. Holland,
Elserver.
24. V.S. Subrahmanian. Hybrid Knowledge Bases for Intelligent Reasoning Sys-
tems, Invited Address, Proc. 8th Italian Conf. on Logic Programming, pps
3-17, Gizzeria, Italy, June 1993.
25. V.S. Subrahmanian. Hybrid Knowledge Systems, Proc. Proc. 7th Space Op-
erations, Applications and Research Symp., pps 218-225, Aug. 1993, Houston,
Texas.
26. C. Bell, A. Nerode, R. Ng and V.S. Subrahmanian. Implementing Deductive
Databases by Linear Programming, Proc. 1992 ACMSIGMOD/SIGACT/SIGART
Symp. on Principles of Database Systems, pps 283-291, San Diego, May 1992.
27. C. Bell, A. Nerode, R. Ng and V.S. Subrahmanian. Implementing Deductive
Databases by Mixed Integer Programming, accepted for publication in ACM
TRANSACTIONS ON DATABASES SYSTEMS.
28. C. Bell, A. Nerode, R. Ng and V.S. Subrahmanian. Implementing Stable Seman-
tics by Linear Programming, Proc. 1993 Intl. Workshop on Logic Programming
and Nonmonotonic Reasoning, pps 23-42, Lisbon, Portugal, June 1993. MIT
Press.
29. Sibel Adali and V.S. Subrahmanian. Intelligent Caching in Hybrid Knowledge
Bases, in: Proc. 1995 Intl. Conf. on Very Large Knowledge Bases, IOS Press,
Twente, The Netherlands, May 1995.
11

30. A. Gupta, I.S. Mumick and V.S. Subrahmanian. Maintaining Views Incremen-
tally, Proc. 1993 ACM SIGMOD Conf. on Management of Data, pps 157-165,
Washington, DC. (with A. Gupta and I. S. Mumick).
31. D. Perlis and V.S. Subrahmanian. Meta-languages, Reflection Principles, and
Self-Reference, in: "Handbook of Logic in Artificial Intelligence and Logic Pro-
gramming", pps 323-358, ed. Dov Gabbay, Oxford Univ. Press, 1994.
32. C. Bell, A. Nerode, R. Ng and V.S. Subrahmanian. Mixed Integer Programming
Methods for Computing Non-Monotonic Deductive Databases, JOURNAL OF
THE ACM, Vol. 41, Nr. 6, pps 1178-1215, Nov. 1994.
33. S. Marcus, V.S. Subrahmanian. Multimedia Database Systems, to appear
in MULTIMEDIA DATABASE SYSTEMS: RESEARCH ISSUES AND DIRECTION,
SPRINGER VERLAG.
34. R. Ng and V.S. Subrahmanian. Probabilistic Logic Programming, INFORMA-
TION AND COMPUTATION, 101, 2, pps 150-201, 1993.
35. S. Kraus and V.S. Subrahmanian. Multiagent Reasoning with Probability, Time
and Beliefs, INTERNATIONAL JOURNAL OF INTELLIGENT SYSTEMS, 10(5) 459-
499.
36. G. Gottlob, S. Marcus, A. Nerode and V.S. Subrahmanian. Non-Ground Stable
and Well-Founded Semantics, submitted to THEORETICAL COMPUTER SCI-
ENCE journal, currently under revision.
37. K. Erol, D. Nau and V.S. Subrahmanian. On the Complexity of Domain-
Independent Planning, Proc. 1992 Conf. of the American Association for Ar-
tificial Intelligence (AAAI-92), pps. 381-386, MIT Press, July 1992.
38. E. Hwang and V.S. Subrahmanian. Querying Video Libraries. Accepted for
publication in: JOURNAL OF VISUAL COMMUNICATION AND IMAGE REPRE-
SENTATION.
39. J. Grant and V.S. Subrahmanian. Reasoning About Inconsistent Knowledge
Bases, IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGINEERING,
7,1, pps 177-189.
40. S. Marcus and V.S. Subrahmanian. Relating Database Updates to Nonmono-
tonic Reasoning. UNIVERSITY OF MARYLAND TECHNICAL REPORT.
41. J. Lobo and V.S. Subrahmanian. Relating Minimal Models and Pre-Requisite-
Free Normal Defaults, INFORMATION PROCESSING LETTERS, 44, pps 129-133,
1992.
42. V.S. Subrahmanian and C. Zaniolo. Relating Stable Models and AI Planning
Domains, in: Proc. 1995 Intl. Conf. on Logic Programming, Tokyo, Japan.
12

43. K.S. Candan, S. Jajodia, V.S. Subrahmanian. Secure Mediated Databases;
submitted for publication.
44. X. Y. Wang, S. Jajodia, and V.S. Subrahmanian. Temporal Modules: An
Approach Toward Federated Temporal Databases, INFORMATION SCIENCES,
Vol. 82, pps 103-128, 1995.
45. K.S. Candan, J. Grant, V.S. Subrahmanian. A Unified Treatment of Null Val-
ues. UNIVERSITY OF MARYLAND TECHNICAL REPORT; submitted to VLDB
journal.
46. S. Adali, R. Emery. A Uniform Framework for Integrating Knowledge in Het-
erogenous Knowledge Systems, to appear in PROCEEDINGS 1995 INTERNA-
TIONAL CONFERENCE IN DATA ENGINEERING, TAIPEI, TAIWAN.
47. J. Benton and V.S. Subrahmanian. Using Hybrid Knowledge Bases for Missile
Siting Problems, Proc. 1994 Conf. on Artificial Intelligence Applications, pps
141-148, IEEE Computer Society.
48. V.S. Subrahmanian, D.S. Nau and C. Vago. WFS + Branch and Bound =
Stable Models, IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGI-
NEERING, Vol. 7, no. 3, June 1995.
49. J. Benton, S.S. Iyengar, W. Deng, N. Brener, and V.S. Subrahmanian. Tactical
route planning: New algorithms for decomposing the map, accepted for pub-
lication in: Proc. 1995 IEEE Intl. Conf. on Tools for Artificial Intelligence,
Washington DC, Nov. 1995.
13

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