summing 5 largest nodes

I am attempting to sum the certain groupings of nodes, based on the max and
min values contained in the nodes.

For example, if there are 20 node elements, sum the 5 largest and the other
15 values (ending with 2 values)

using POSITION(), I can determine what the 5 largest (or 15 other values
are), but I can't seem to determine how to sum these values.

my XML looks like

<items>
<item>
    <value>6</value>
</item>
<item>
    <value>3</value>
</item>
<item>
    <value>7</value>
</item>
</items>

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