tokens: I am grateful to T. Baker and M. G. Santos for extensive discussions, and acknowledge Y. Akrami, S. Camera, P. G. Ferreira, A. Raccanelli, J.-P. Uzan, M. Viel, F. Villaescusa-Navarro, and K. Zarb-Adami for useful comments and suggestions. I was supported by European Research Council grant StG2010-257080. The code and Fisher matrices used in this paper are available from http://philbull.com/mg_ska/ .
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