6 Acknowledgements

It is a pleasure to thank Wes Colley, Frederic Courbin, Emilio Falco, Henk Hoekstra, Neal Jackson, Tomislav Kundić, Geraint Lewis, and Andrzej Udalski for permission to use their figures. I would also like to thank Matthias Bartelmann, Emilio Falco, Jean-Paul Kneib, Bohdan Paczyński, Sjur Refsdal, Robert Schmidt, Liliya Williams, and David Woods for their careful reading of (parts of) the manuscript at various stages and their useful comments. Of particular help were the comments of Jürgen Ehlers and an unknown referee which improved the paper considerably.

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