The XPath expression that you have written merely matches the outermost node.   What you might want to use here are axes.

Try something like this:   (extemporaneous answer, check it yourself)

//xml:msub[count(descendant::xml:msub) = 0] to retrieve all msubs who do not have any descendants of the same type.   These must be the leaves of the structure.

Now, for each of these, perform a separate query for ancestor::xml:msub which will return an ordered list (from closest to farthest, or is it the other way around?) of all ancestors back to the top of the tree.