NET-CLAUSE LANGUAGE (NCL)
        Written by Zdravko Markov, Bulgarian Academy of Sciences
                      Shelved on 20th August, 1993


Net-Clause Language (NCL)
=========================

NCL is aimed at describing distributed computation models using term
unification as a basic processing and control mechanism. NCL is embedded
in standard Prolog and comprises two parts, which can communicate each
to other and to standard Prolog programs through the unified
representation of terms, the unification procedure and the database
mechanism provided by Prolog.

Unlike most of the entries in this library, NCL is not purely a Prolog
program. Instead, it is an augmented Prolog interpreter written in C,
together with some Prolog library routines. So you need C in order to
run NCL.                


1. Net-Clauses
--------------

A net-clause is a special domain in the database defining a network of
nodes and links. The nodes are represented by Prolog compound terms. The
variables occurring within the nodes are global logical variables, which
can be shared within the scope of the net-clause thus playing the role
of network links. The net-clause nodes define local conditions for
unification of terms or execution of Prolog goals. Thus two basic
mechanisms are implemented:

 - Spreading activation. This is a distributed computation scheme
   similar to the connectionist spreading activation or to the marker
   passing mechanism in SN's. In the framework of logic programming it
   is seen as a restricted (without recursion) forward chaining
   computation.

 - Default mechanism similar to the Reiter's default assignment to
   variables. The basic idea is to use variables to propagate terms
   without being bound to them, thus implementing the non-monotonicity
   of default reasoning.


2. Data-driven Rules (DD-rules)
-------------------------------

DD-rules implement a full scale forward chaining for logic programs.
They also simulate a data-driven parallel computation, where each
DD-rule is a process (in contrast to the traditional parallel logic
programming, such as PARLOG for example, where each goal is a process).


NCL/Prolog Interpreter
======================

NCL/Prolog interpreter is supplied in C-code for UNIX along with a
reference manual and a set of annotated examples.

The rest example programs each contain detailed comments, and are as
follows:

NLP     - Natural language parsing and semantic analysis
DEFAULT - NL parsing by using NCL default mechanism
GRAMMAR - Grammar rules by DD-rules
MP      - Marker passing in semantic nets
CIRCUIT - Model-based diagnosis of a binary adder
QUEENS  - 4-queens problem as a network of communicating agents
FIG     - Recognition of geometric figures
XOR     - Connectionist-like implementation of XOR
PIXEL   - Connectionist-like feature detection in a binary image
FLATTEN - Stream AND-parallelism in DD-rules
DDIL    - Data-driven inductive learning in a network of relations
FAMILY  - Set of training examples for DDIL


Address for further information
===============================

Zdravko Markov
Institute of Informatics, Bulgarian Academy of Sciences
Acad.G.Bonchev Street, Block 29A, 1113 Sofia, Bulgaria
Tel: +359-2-707586
Fax: +359-2-720166
Email: markov@iinf.bg


SIZE: 210 kilobytes.


CHECKED ON STANDARD C : no.


PORTABILITY : Haven't had time to check.                 


INTERNAL DOCUMENTATION : No comments in the C source code. The
user manual gives some idea of how the NCL-specific parts of the
interpreter work, but you will need to know how Prolog interpreters are
implemented to understand the source properly.