Some R packages provide generic functions e.g. for training models, calculating distances, etc., where one model out of a set of various possible models has to be chosen as a parameter of the function.

Issues:

• Documenting the possible entries is not easy, because a single model description already contains a lot of information. When many models must be described in help, the help page of the function becomes to large (in particular the parameter description part).
• The model description itself should be structured.

How is this problem solved by different popular packages?