XPath: selecting matching nodes in two node-sets

I have the following document:

<results>
        <keywords>
                <matches>...</matches>
                ...
        </keywords>
        <item>
                ...various elements...
                <matches>...</matches>
                <matches>...</matches>
                ...
        </item>
        <item>....</item>
        ...
</results>

(where <matches> elements contain the keywords themselves, CDATA).

and what I need is an XPath expression which selects all the <item>s which 
have the same set of <matches /> elements as in the <keywords> node.
(Note: there may be repetition of <matches />s in <item>s but not in 
<keywords>)

I've tried:
"//item[matches = //keywords/matches]"
but this selects <item>s where not all the '//keywords/matches' elements are 
present.

Cheers,
Richard

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