 We are grateful to David Miller, Sally Shrapnel, Rod Sutherland, and Ken Wharton for helpful discussion and comments on earlier versions. P.W.E. acknowledges the support of the University of Queensland and the Australian Government through the Australian Research Council’s Discovery Early Career Award scheme (DE170100808). S.F. acknowledges the support of the Netherlands Organization for Scientific Research (NWO), Veni grant 275-20-065.