Thanks for responding, both, it is particularly great to get an answer from the father of mixed models in R and julia!
I do have to say I find the functionality slightly disappointing, though. I had done what was in the suggested solution, but thought of it as an unpleasant workaround. Can anyone explain why Julia has chosen such an interface? I can think of no cases where it is a feature, and I see it as a problem in several ways, e.g. it does not generalize the functionality and the data as separate modular entities, which also means that the formula does not express the actual relationship modeled. Also, it does not allow a model object that can predict on the basis of a single input variable. I realize the result will be the same, but in my experience julia is also concerned with elegance and intuitive specification. I will say I am really surprised. Is this just a matter of implementation, so a possible @formula macro planned for the future will restore this functionality? I have no strong feelings about then '1 +' , it will require typing a few unneccesary extra characters, but perhaps there is a pedagogical gain from requiring people to specify that they want an intercept fitted as well.
