xsl group by with when date to equals date from

I'm using xslt 2.0 for transformation to a new XML This is my xml Structure:

<?xml version="1.0" encoding="UTF-8"?>
<bills>
    <bill id="1">
        <rz_from>05.11.2011</rz_from>
        <rz_to>31.12.2011</rz_to>
        <rz_price_pro_unit>4</rz_price_pro_unit>
        <rz_units>7</rz_units>
    </bill>
    <bill id="2">
        <rz_from>1.1.2012</rz_from>
        <rz_to>31.3.2012</rz_to>
        <rz_price_pro_unit>4</rz_price_pro_unit>
        <rz_units>9</rz_units>
    </bill>
    <bill id="3">
        <rz_from>1.5.2012</rz_from>
        <rz_to>31.12.2012</rz_to>
        <rz_price_pro_unit>4</rz_price_pro_unit>
        <rz_units>21</rz_units>
    </bill>
    <bill id="4">
        <rz_from>1.1.2013</rz_from>
        <rz_to>31.12.2013</rz_to>
        <rz_price_pro_unit>5</rz_price_pro_unit>
        <rz_units>45</rz_units>
    </bill>
    <bill id="5">
        <rz_from>1.1.2014</rz_from>
        <rz_to>31.12.2014</rz_to>
        <rz_price_pro_unit>5</rz_price_pro_unit>
        <rz_units>51</rz_units>
    </bill>
</bills>

I need to group the bill nodes by these rules:

• the price per unit is the same


• the date (rz_from - 1 day) is equal to the date (rz_to) of the previous node


• the count of units should by the sum of the grouped values

So the result should be:

• Bill: 16 units - price 4 (bill 1+2)


• Bill: 21 units - price 4 (bill 3)


• Bill: 96 units - price 5 (bill 4+5)