[
 []              ,
 [ [] ]          ,
 [ [] , [ [] ] ]
]
##</code><code>##

sub ordinal {
    my @list = defined($_[0]) ? @{ $_[1] } : ();

    if ( $_[0] == 0 ) { return  \@list            }
    if ( $_[0] == 1 ) { return [ @list , \@list ] }

    return ordinal( $_[0] - 1 , [ @list , \@list ] );
}