Attribute Insertion behavior

I have a question about the insertion behavior of an attribute in an
element's attribute list. 
Assuming I have an element 'e' and an attribute 'a' named "A" with value
"old", what is the expected behavior supposed to be in the following case?

e.setAttributeNode(a);
a.setValue("new");
Attr a2 = e.getAttributeNode("A");

Is a2's value supposed to equal "new" or "old".  Or is this strictly an
implementation decision?

Thanks,

Blaine Brodie