Multiple instance elements and default for binding

Hi,

can anybody please give me a hint where the specification defines the 
behaviour of using the single node binding attribute "ref" without using 
"instance('xyz')" in the path expression if the model has multiple 
instance children?

One obvious choice would be to use the first instance element in the 
model. Another could be to try to match the path against all instance(s) 
until a match is found. This, I assume, would have been mentioned in 7.3 
"Evaluation Context", so I doubt that this is intended. But I couldn't 
find confirmation of the first alternative neither...

Thanks

  - Michael