At the moment we want to just replace SDF. Have the exact same thing so we can also test.
Also, I don't think this is something that we want to support. We would like to have a simple and easy to explain mechanism. Maude fails in this regard. It's hard to explain what gather does, and you always  have to check to manual to make sure you did it right.

The more complex productions, like your example, could be handled by the rewrite engine which takes fully general disambiguation rules.