processin each element not on a list

I have xml in substantially the following form:

<root>
 <choices>
  <choice>A</choice>
  <choice>B</choice>
  <choice>C</choice>
  <choice>D</choice>
 </choices>
 <items>
  <item id="1">A</item>
  <item id="2">X</item>
  <item id="3">B</item>
  <item id="4">C</item>
  <item id="5">Y</item>
  <item id="6">Z</item>
 </item>
</root>

I need to process each item where its content is not one of the
choice values. I need a very efficient solution since I have say
up to 10,000 <items>s and perhaps 100 difference <choice>'s. 

Thanks in advance.

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