A node matches a pattern if the node is member of the result of
evaluating the pattern wrt. some context.
(Compare this with XPointer use of XPath.)
|pattern:||location path | ... | location path|
|location path:||/step/ ... // ... /step|
|step:||axis nodetest predicate|
matches subsection elements occuring either as child elements of section elements or as descendants of appendix elements.
(The whole XPath language is used for expressions later...)