The authors are obliged to C. Eaton, J. Farrell, D. Fudenberg, R. Masson, J. Panzar, C. Shapiro, P. Regibeau and R. Willig for comments on earlier portions of this work. The usual disclaimer applies. Much of this work relies on Geroski and Jacquemin (1984), Gilbert (1989a), and Geroski (1983).
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