Defining a method in two packages

I think a simpler solution, which I have implemented in lme4 but not
yet released is to have

importFrom(nlme, ranef, fixef)

in the NAMESPACE file of packages that implement methods for those
generics (and, of course, add nlme to the Imports: specification in
the DESCRIPTION file).  As nlme is a required package I don't think
this is too much of a burden.

I'll do the same change in coxme.  

We can save Kevin Coombe's solution of a separate package of "method
stubs" (which I find very similar in spirit to a shared .h file in C)
for the day that ranef/fixef gets wildly popular.  An importMethods
directive for the namespace is an intriguing long term idea for the core
team to ponder. 

Again, thanks fo all the input.  This discussion has been very
illuminating for me.

Terry T.