sub mapn (&@) {
    my ( $callback, $n ) = splice @_, 0, 2;
    my ( $i, $j ) = ( 0 ) x 2;
    map {
        $i = $j;
        $n = @_ - $i if $i + $n > @_;
        $callback->( @_[ $i .. ( $j += $n ) - 1 ] );
    } 0 .. @_ / $n - 1;
}