sub allPrefs
{
    my( $x, $bits, $y )= @_;
    $Pref->{$x}{$x}= EQU();
    onePref( $x, $bits, $y );
    while(  my($z,$v)= each %{$Pref->{$y}}  ) {
        onePref( $x, $bits & $v, $z );
    }
}

sub partialOrder
{
    for my $chain (  @_  ) {
        while(  $chain =~ /\G\s*(\w+)\s*([<=>])\s*(?=(\w+))/gc  ) {
            my( $x, $c, $y )= ( $1, $2, $3 );
            my $bits= $OpBits{$c};
            allPrefs( $x, $bits, $y );
            allPrefs( $y, $RevBits{$bits}, $x );
        }
        if(  $chain !~ /\G\s*(\w+)\s*$/g  ) {
            die "Invalid preferences ($chain).\n";
        }
    }
}