XSL equivalent to SQL "unique"

Hi,

I've spent some time to figure out an easy way to solve the following
problem and failed, maybe some Guru can give me a clue?

Having XML input like

	<set name="1">
		<a>v1</a>
		<c>v2</c>
		<d>v3</d>
	</set>
	<set name="2">
		<a>v4</a>
		<b>v5</b>
		<e>v6</e>
	</set>
	<set name="3">
		<b>v7</b>
		<f>v8</f>
	</set>

I need to render a quadratic schema like

		a 	b 	c 	d 	e 	f 
	1	v1		v2	v3
	2	v4	v5			v6
	3		v7				v8

in HTML, i.e., I need output like

	<table>
	
<tr><td></td><td>a</td><td>b</td><td>c</td><td>d</td><td>e</td><td>f</td></t
r>
	
<tr><td>1</td><td>v1</td><td></td><td>v2</td><td>v3</td><td></td><td></td></
tr>
	
<tr><td>2</td><td>v4</td><td>v5</td><td></td><td></td><td>v6</td><td></td></
tr>
	
<tr><td>3</td><td></td><td>v7</td><td></td><td></td><td></td><td>v8</td></tr
>
	</table>

Of course all dimensions are variable dependand on underlying
data. I thought, the problem boils down to obtaining the a list like

	<n>a</n><n>b</n><n>c</n><n>d</n><n>e</n><n>f</n>

in a variable from the initial input. But I failed to obtain that, though
I'm fairly sure it has to "be there" :-(

Any help?

Thank you very much,
Frank

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