=head1 NAME

Logic3 - Perl module providing 3-value logic versions of 
         "and", "or", "xor" and "not".

=head1 SYNOPSIS

	use Logic3 qw( And Or Xor Not AND OR XOR NOT TRUE FALSE UNDEF ) ;

	use Logic3 ':ALL' ;

	use Logic3 ':Constants' ; # Gets constants TRUE FALSE UNDEF only.

	use Logic3 ':Propagating' ; # Gets constants and propagating functions.

	use Logic3 ':NonPropagating' ; # Gets constants and non-propagating funcs.

These functions are for scalars, literals and constants. (They don't
dereference so there's no point in sending them references since
references are always true.)

    # These propagate undefined.
    $result = And( $a, $b ) ;
    $result =  Or( $a, $b ) ;
    $result = Xor( $a, $b ) ;
    $result = Not( $a ) ;

    # These do not propagate undefined.
    $result = AND( $a, $b ) ;
    $result =  OR( $a, $b ) ;
    $result = XOR( $a, $b ) ;
    $result = NOT( $a ) ;

=head1 DESCRIPTION

Perl's built-in logical operators, C<and>, C<or>, C<xor> and C<not>
support two-value logic. This means that they always produce a result
which is either true or false. In fact perl sometimes returns 0 and
sometimes returns undef for false depending on the operator and the
order of the arguments. For "true" Perl generally returns the first 
value that evaluated to true which turns out to be extremely useful 
in practice. Given the choice Perl's built-in logical operators are 
to be preferred - but when you really want 3-value logic it is 
available through this module.

Three-value logic with the logical operators, C<And>, C<Or>, C<Xor>,
C<Not> and C<AND>, C<OR>, C<XOR>, C<NOT> provided in this module produce
one of three results: true, false or undefined. They are "pure" logical
operators in that they always return one of the constants, TRUE
(1), FALSE (0) or UNDEF (undef) rather than one of their inputs.

Three-value logic has two different truth tables for "and" and "or";
this module supports both. (In the Perl column F means F or U.)

Note that C<And>, C<Or>, C<Xor>, and C<Not> propagate undefined and
C<AND>, C<OR>, C<XOR>, and C<NOT> don't.

        Perl Logic3         Perl Logic3        Perl Logic3 
    A B and  And AND    A B or   Or  OR    A B xor  Xor XOR
    - - ---  --- ---    - - --   --  --    - - ---  --- ---
    U U  F    U   U     U U  F    U   U    U U  F    U   U 
    U F  F    U   F     U F  F    U   U    U F  F    U   U 
    F U  F    U   F     F U  F    U   U    F U  F    U   U 
    F F  F    F   F     F F  F    F   F    F F  F    F   F
    U T  F    U   U     U T  T    U   T    U T  T    U   U
    T U  F    U   U     T U  T    U   T    T U  T    U   U
    T T  T    T   T     T T  T    T   T    T T  F    F   F
    T F  F    F   F     T F  T    T   T    T F  T    T   T
    F T  F    F   F     F T  T    T   T    F T  T    T   T

         Perl Logic3
    A not  Not NOT
    - ---  --- ---
    U  T    U   U
    U  T    U   U
    F  T    T   T
    T  F    F   F

=head1 BUGS

Although we can use the following comparisons:
    ( $result == FALSE )
and
    ( $result == TRUE )
using
    ( $result == UNDEF )
will lead to complaints if you use -w (which you should of course), if
$result happens to be undefined, so the last comparison should be
re-written as follows:
    ( not defined $result )

=head1 CHANGES

1998/4/27   First release.

1998/6/25   First public release.

1999/01/18  Second public release.

1999/02/26  Changed TRUE, FALSE, UNDEF to standard perl-style constants.

1999/07/30  No code changes. Changed for CPAN and automatic testing.

1999/08/08  Changed licence to LGPL.

=head1 AUTHOR

Mark Summerfield. I can be contacted as <summer@chest.ac.uk> -
please include the word 'logic' in the subject line.

=head1 COPYRIGHT

Copyright (c) Mark Summerfield 1998/9. All Rights Reserved.

This module may be used/distributed/modified under the LGPL. 

=cut