sub kronecker_product {
    my ($x, $y) = @_;
    my ($x0, $x1) = $x->dims;
    my ($y0, $y1) = $y->dims;

    return (
        $y * $x->dummy(0, $y0)->dummy(1, $y1)
    )->xchg(1, 2)->reshape($x0 * $y0, $x1 * $y1)
}