Support for quickly classifying or annotating with labels from a hierarchy

[Sven <hartrumpf@gmail.com>]

Hi all.

I have a fixed classification comprising around 50 classes.
The classes are organized in a simple hierarchy (tree depth up to 3 or
4).
How can I efficiently classify lines in a file (or to put it
differently:
annotate the objects represented in each line)?
The root node of the classification is labeled top here.
The classification could be stored in a separate file
(classification file) like this:
--------------------------
top
 cl1     ; (cl1 is direct subclass of top)
  cl1.1  ; (cl1.1 is direct subclass of cl1)
  cl1.2
 cl2
  cl2.1
  cl2.2
 cl3
 cl4
  cl4.1
   cl4.1.1
   cl4.1.2
  cl4.2
--------------------------
Here, subclasses are indented, but any format would be ok because
the classification is fixed. The classification should be visible
(maybe browsable, clickable, keyable) when one initiates
classification
of a line (say, on the line "Rome top" below).

Example annotation file:
--------------------------
Berlin top
Paris top
Rome top
--------------------------

I want to _efficiently_ (in terms of human annotation speed) modify
the
"annotation file". In the example above, the annotation file could
look like this:
--------------------------
Berlin cl1.2
Paris cl3
Rome cl4.1
--------------------------

Mouse-oriented and keyboard-oriented solutions welcome, but in both
cases the hierarchy should be visible when classification of a line is
initiated.

Any emacs extensions that will help?

Greetings
Sven