<workbook>
<worksheet name="backref">
<cell ref="foo" text="1" />
<cell ref="foo" text="2" />
<cell ref="foo" text="3" />
<cell>=SUM(<range ref="foo"/>)</cell>
</worksheet>
</workbook>
<workbook>
<worksheet name="backref">
<cell ref="foo" text="1" />
<cell ref="foo" text="2" />
<cell ref="foo" text="3" />
<cell>=SUM(<range ref="foo"/>)</cell>
</worksheet>
</workbook>