9 Acknowledgments

I am very thankful to Steve Saar for his careful reading and thoughtful comments on the manuscript, and to Karel Schrijver and anomymous referees for very useful comments and their help to shape this article. Financial support is acknowledged from the Deutsche Forschungsgemeinschaft through the Emmy Noether programme, RE 1664/4-1, and the Heisenberg Programme, RE 1664/9-1.

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