append values to a String

Hi,

I had a similar question to the one Stevenson just asked about variable.  I
know it's impossible to re-assign value to a variable in XSL, that's why I
don't know what to do in here.  

Here's the xml:

<Sample>
	<K1/>
	<K2>something</K2>
	<K3>something else</K3>
	<K4/>
</Sample>

I'd like to loop through all the children nodes under <Sample>, find those
have non-empty value, and construct a String with all those values append to
each other with a "," delimiter in between them, i.e. the result String
should look like:  "something,something else,".  And I need to assign this
String to a variable since I need to use it elsewhere.

There might be more than 80 children nodes under <Sample>, that's why I
don't want to hard-code concat() in this case...

Any suggestion is highly appreciated!

Thanks,
Dongzhi

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