[Bug 4793] Restructure section 3.4 to follow the following 1) mapping from syntax to cannonical representation, 2) what constitutues a valid usage and 3) implications on instances.
[Bug 4802] For bullet points 4,5 (...XPath expression must conform to...) we should remove the BNF and refer to the Schema spec section instead and add that the xpath expression can also include the deref() function