[Users] prolongation, interpolation

I'm looking for a very simple prolongation strategy, if two points exactly coincide the copy the values. If it is between, assign the average of the 2 (or 4) neighboring points. How do I get this behavior? I tried various interpolators (is it "copy"?), some work and some don't. If I try to use "copy", I get an error:

There is no vertex-centred stencil for op="LAGRANGE" or op="COPY" with order_space not in {1, 3, 5, 7, 9, 11}

I am setting prolongation_order_space = 1, however. I assume it's the vertex centering that's the problem. Thanks.

Cheers, Steve