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Published online by Cambridge University Press: 16 March 2015
Figure 3 was left out in the article (Gottlob et al. 2013). The figure that is given there as figure 3 should in fact be figure 4. Further, the reference to figure 3 on the 4th line of page 889 should be a reference to figure 4. The correct figure 3 (missing in the paper) is supplied below. We apologise for this error.