Sorry about the confusion with old manuals etc.
The only files you need to change are in wspec/. You don't need to
touch any *.c and *.h files in other directories. They are for
programmer convenience to define C handles for tags used by the code.
You do not need to recompile if you want to change the schema.
To add a new class, add a line to wspec/options.wrm, and define a new
corresponding model in wspec/models.wrm.
To add new tags to a model, just add them in models.wrm. The only
constraints are that the tag should look like a standard identifier
(fitting regexp [A-Za-z][A-Za-z0-9_]*) and a tag can not be used more
than once in any one model.