JAXB: umarshaling mixed xml elements

I have a similar problem. I have a similar XML, like this:

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <y>1</y>
    <x>A</x>
    <x>B</x>
    <x>C</x>

    <y>2</y>
    <x>D</x>
    <x>E</x>
    <x>F</x>

    <y>3</y>
    <x>I</x>
    <x>J</x>
    <x>L</x>
    ...
</root>

But, I need that my result to be:

Object root
    List<SetNumber> entries
        - y
        - List<X> x;

I need to get together the set of <y> and <x>'s (in this case, three elements) in a new element (for example, SetNumber).

Another problem is that the list of <x> is dynamic. The XML can take any amount of (greater than one).