[Gretl-users] efficient duplication matrix?

Hi, I was wondering what's an efficient way to calculate a (pre-) multiplication with the duplication matrix. (For a definition of what I mean see here: https://en.wikipedia.org/wiki/Duplication_and_elimination_matrices or here: https://www.rdocumentation.org/packages/matrixcalc/versions/1.0-3/topics/...) It's of course very easy to construct that matrix explicitly and then do the multiplication, and I already have a function for that, but I suspect it's not very efficient. (Similar to the commutation matrix, where in the 'extra' addon we have a function called 'commute' which achieves that without actually creating the interim matrix.) So, any clever ideas? thanks sven