> Do they just make 
> a string with some kind of separator breaking up the attributes?
> Isn't there a chance of this failing if the character used 
> for the separator is used in one or more of the attributes?
> 

Yes, and yes.

Unicode gives a wide range of characters to choose from, including the
Private Use area.

An alternative approach is to create separate keys one for each attribute,
and then to search using

key('ka', @a) intersect key('kb', @b) 

Regards,

Michael Kay
http://www.saxonica.com/
http://twitter.com/michaelhkay