XPATH: Set 'remove-all-values'

I am trying to build a policy which checks for a range of attributes if
there is a modify available. if so, I would like to change the event so
whatever comes in, the output should always be:

<modfy-attr attr-name="X">
<remove-all-values>>
<add-value><value>New Value</value><add-value>
</modify-attr>

Where the input might have been:

<modfy-attr attr-name="X">
<remove-value><value> old</value></remove-value>
<add-value><value>New Value</value><add-value>
</modify-attr>


Background is that the destination is not a multi-value attribute and I dont
want to have specific removes (based on the value of the attribute)

The XPATH should be able to use a '|' operator, so something like
modify-attr["@attr-name='X'|"@attr-name='Y'|@attr-name='Z']

Does someone alrady have a similiar code built?

Thx,
Thorsten