A new facility has been committed for version 2.8.0 that allows S4 generics to be defined with "..." as the signature. Methods are then defined with a single class in the signature. The interpretation is that the method will be selected and called when all the arguments matching "..." are from the specified class, or some subclass of that class. See ?dotsMethods for details. The current implementation is a test. It's dispatch is done in R code, as described in the documentation. Once the desired semantics are clear, it's a natural candidate for adding to the current C dispatch code. Also, in this version "..." cannot be combined with other arguments in the generic function's signature. This is also not a total necessity and might be relaxed, after the dispatch is rewritten. Please play with it, if it sounds useful. John