parsers.ml has now good potential for refactoring...
functions reordered by functionality; some comments added.
Again use String.trim to remove the last added "\n".
extraction-functions for arg-keys.-/vals/pairs
"argpair" renamed to "argpairs"  `Args removed
tagselect-Functionality implemented. Semantics will change again soon!

At the moment only the first element of a doclist is shown.
Change will be: *each* top-element of the selected items in the doclist
must be shown (dump) and put into the result.

The reason is: the selected elements are always the TOP-elements,
which matched the selector-function. And all stuff below it is in the subnodes.
These will be displayed by dump.
Dump dumps in-depth as well as *each* item in the tagselection.

It must be distincted between traversing each selected node in depth
(looking at the subnodes)
and traversing the selected doclist.
The selected stuff: always Elemtn(_,_,_).
dump_html shows all of them,
but the extractors don't do that at the moment.
This is the next needed change.