[From open in issue in Appendix E]

Implementation of references that target non-root elements is challenging for persistent SML stores built on top of relational database systems. The standards body to which SML gets submitted should investigate this and, if needed, explore options to ease the implementation burden for persistent SML stores using relational databases. Possible areas of investigation are 

Restricting the XPath expression in XPointer fragment (similar to XPath expressions allowed in xs:selector)

Defining a new scheme for URIs that contain XPointer fragment