add more static typing to the source

[bansp]

Frequently, the names of variables in the source switch between singular and plural, and sometimes their reference to the plurality of the requested element name is not clear either (is it a single item, or a sequence, etc.). See the name "recommendation(s)", which typically requires me to jump all around the source tree, because as long as our backend is eXist, I am not able to push the code through the debugger (it is constantly in a bugged state, cf. #177 )

I'd like this issue to be a reference point, at least for a while.