Grouping elements by attribute values

HI,
Help Please.

I have a set of foo elements. Each foo element has an rl attribute and may
have a wl attribute.
eg.

<foos>
	<foo rl="C" wl="W">CW1</foo>
	<foo rl="C" wl="W">CW2</foo>
	<foo rl="B" wl="W">BW1</foo>
	<foo rl="A" wl="W">AW1</foo>
	<foo rl="C" >C1</foo>
	<foo rl="C" >C2</foo>
	<foo rl="B" >B1</foo>
	<foo rl="A" >A1</foo>
	<foo rl="C" >C3</foo>
</foos>

I want to transform the set such that:

a.  For each unique set of rl and wl attributes there is only one element.
b.  The element values are tramsformed into an NMTOKENS list in the element
with the appropriate attribute set.

ie. to:

<foos>
	<foo rl="C" wl="W">CW1 CW2</foo>
	<foo rl="B" wl="W">BW1</foo>
	<foo rl="A" wl="W">AW1</foo>
	<foo rl="C" >C1 C2 C3</foo>
	<foo rl="B" >B1</foo>
	<foo rl="A" >A1</foo>
</foos>

Peter Sparkes


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