how to select nodes based on attribute values

is there a way to select a node based on attribute value matching attribute
value in another node?

example

<?xml version="1.0" encoding="UTF-8"?>
<xxx>
<a element_id="1" name="xxx"/>
<a element_id="2" name="yyy"/>
<b pick_id="1"/>
</xxx>

i want to select all 'a' elements that have element_id matching the value of
'b's pick_id (my context is being "in" b)
i've tried [] but i can't see how to refer to the attributes of "a" and "b"
simultaneously.

thanks for any help,
archie

archie russell
doubletwist.com
510-587-5766


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