Yeah, I think that might be better.  If that were done that there wouldn't *currently* be any models with parameters using this "optional" argument.  I'm sure other uses for it could be found.

The name "optional" I think might be a bit misleading too.  Currently one can make a parameter optional in the sense that it has a default value, and does not need to be explicitly given a value when creating an instance of that model.  So now we have two senses in which a parameter is "optional".  I think maybe "alternative" might be better, and it makes me wonder if there shouldn't be a way for a model to have one or more alternative sets of parameters...