>  From this line I understand that the XPath 2.0 "=" operator 
> allows to test an atomic value (like the string that the 
> attribute value is coerced into) against a sequence and 
> returns true if the atomic value equals any of the members of 
> the sequence.
> 
> If this is true, where I can find more information about this 
> feature and its prerequisites? I have searched the relevant 
> W3C specifications up and down, but in vain.

http://www.w3.org/TR/xpath20/#id-general-comparisons

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