xforms-binding-exception

Hi,

The spec says that an xforms-binding-exception will be thrown for "an 
illegal binding expression".  Could someone please define this for me, 
please?  I assume this covers the rules defined for 
xforms-compute-exception (i.e. syntax error in the expression, undefined 
namespace used, etc).  Anything else?  If the author writes a binding 
expression in such a way that a non-node type is returned (i.e. uses 
ref="position()"), this does not qualify as an illegal binding 
expression, does it?  In such a case doesn't the processor just ignore 
the element and its children and move on?

Sorry if this is a simple question.  Been away from xforms for a while 
and couldn't find anything on the web that says what happens when a 
binding expression returns a type other than a node-set.

Thanks,
--Aaron