Could you elaborate a little on this paragraph from Section 3.3: "The main aspect of the analysis is determining combinations of axis steps that are “naturally sorted”. This is the case for any sequence of child axis steps. It is also true for an expression such as a/b//c, but not (perhaps surprisingly) for a//b/c. There is no space here to give the rules in detail." ? Perhaps an example XDM tree to demonstrate why a//b/c is not naturally sorted would suffice.