Acknowledgements

Acknowledgements

I received useful comments on a draft of this paper from a referee and my colleagues, Francesco Figari, Xavier Jara, Chrysa Leventi, JekaterinaNavicke, Alari Paulus and Iva Tasseva. I am also grateful to them and to Silvia Avram, Paola De Agostini, Christine Lietz, Olga Rastrigina and Alberto Tumino for permission to draw on our joint work. The usual disclaimers apply.