% % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % %
%                                                                           %
% File:            clogic.dtr                                               %
% Purpose:         define logic for working with constraints                %
% Author:          Lionel Moser, December  1991                             %
% Email:           moser@bnr.ca                                             %
% Documentation:   Cognitive Science Research Paper CSRP 216, Univ. Sussex  %
% Related Files:   arglogic.dtr, arglists.dtr, clogic.dtr, sets.dtr         %
% Version:         7.01                                                     %
%                                                                           %
%      Copyright (c) University of Sussex 1991.  All rights reserved.       %
%                                                                           %
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# load 'arglists.dtr'.
# load 'arglogic.dtr'.

% Moser, Lionel, "Lexical constraints in DATR," Cognitive Science
%   Research Paper CSRP 216, University of Sussex, Brighton,
%   1992.
% 
%   DATR contains no special features to support testing of
%   equality, negation, disjunction, or multiple inheritance.
%   Nevertheless, given an appropriate interpretation it is
%   possible, within DATR's existing syntax and semantics, to
%   represent these operations.  In this paper we review the
%   technique known as `negative path extension', and show how
%   it can be used to reconstruct negation, disjunction, and
%   equality testing.  We then show how these operations can be
%   used to define what are essentially meta-level constraints
%   on DATR lexical derivation.

% Variable $terminal must be defined elsewhere if loading this file from some
% other file.

% # vars $terminal: alpha beta gamma 1 2 3 undef.

% Polyadic AND
% And:<bool bool ... bool ;> ==
%                   true   -  if all bools are true;
%                   false  -  if any bools are false.

And:
    <> == '**** ERROR: (And) Invalid argument'
    <;> == true
    <true>   == <>
    <false>  == false.

% Polyadic OR
% Or:<bool bool ... bool ;> ==
%        true  -   if any bool is true;
%        false -   if no bool is true.

Or:
    <> == '**** ERROR: (Or) Invalid argument'
    <;> == '**** ERROR: (Or) Nil bool list'
    <true ;> == true
    <false ;> == false
    <true>   == true.

% Negation

Not:
    <> == '**** ERROR: (Not) Invalid argument'
    <true> == false
    <false> == true.

% Implication

Implies:
    <> == true
    <true false> == false.

% We assume a default value of 'undef' for any path lacking a value
% within the descriptive domain of the theory.
% A path is defined if it does not extend 'undef'.

Is_defined:
    <undef> == false
    <> == true.

% Safe:<path> 
% A path is `safe' if it satisfies its constraints when evaluated with
% respect to the global context. If the path is safe, then return this
% value; if it is not safe, return a message.

Safe:
    <> == <If:<"<constraints>">>
    <then> == "<>"
    <else> == 'constraint violation'.

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% The next line is the Revision Control System Archive Id: do not delete it.
% $Id: archive.dtr,v 1.1 1997/04/09 20:40:33 root Exp $