Group elements based on summing values (XSL 1.0)

Hello,

My XSL skills are a bit rusty.

Starting from the first <num> element, I need to create the smallest group
possible where the sum of the <num> elements totals 10 or more. I am only
able to use XSL 1.0.

INPUT:
<row>
    <num>3</num>
    <num>4</num>
    <num>4</num>
    <num>7</num>
    <num>12</num>
    <num>15</num>
    <num>1</num>
</row>

OUTPUT:
<row>
    <group1>
        <num>3</num>
        <num>4</num>
        <num>4</num>
    </group1>
    <group2>
        <num>7</num>
        <num>12</num>
        <num>15</num>
        <num>1</num>
    </group2>
</row>

Any help/pointers appreciated.

--
Kevin