Acknowledgments

We thank Anno Kürth, Aitor Morales-Gregorio, Günther Palm, Moritz Helias, Jan Bölts, Abel Jansma, Thomas Nowotny and Manfred Opper for helpful comments and discussions.

Funding

This work was partly supported by the German Federal Ministry for Education and Research (BMBF Grant 01IS19077A), the Canadian National Research Council (NSERC) and the government of Ontario (OGS).

A.R developed the theory, wrote the software implementations and the first draft of the manuscript. A.R and A.L. discussed the results and revised the manuscript.