sub turn_tables {
    my ( $pdl, $dir ) = @_;     # dir = 0   => forward
                                #       1   => backward
    my @t = (   # transform
        sub { $_[0]-> copy-> transpose-> slice( '-1:0, X' )},   # 90
        sub { $_[0]-> copy-> slice( '-1:0, -1:0' )},            # 180
        sub { $_[0]-> copy-> transpose-> slice( 'X, -1:0' )},   # 270 
    );
    @t = reverse @t if $dir;

    for ( 1 .. 3 ) {
        my $s = $pdl-> slice( "X, X, $_" );
        $s .= $t[ $_ - 1 ]-> ( $s )
    }
}