tokens: ACKNOWLEDGEMENTS JWM dedicates the work in this paper to his father, William McKee. We thank Francis Graham-Smith, Gemma Janssen, Benjamin Shaw, Patrick Weltevrede, and Cristina Ilie for very helpful discussions, and the anonymous referees for useful comments. The authors acknowledge the support of their colleagues at Jodrell Bank Observatory. This work was supported by the UK Science and Technology Research Council, under grant number ST/L000768/1.
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