MINIMUM DIFFERENTIATION. Truck and Barter takes up the perpetual discount problem and the minimum differentiation principle by identifying minimal clustering of rug merchants in his neighborhood. The general topic might reward further study; in fact there might be a dissertation or so lurking in it. Readers might want to look at Jean-Claude Thill, "A Note on Multipurpose and Multistop Shopping, Sales, and Market Areas of Firms," an extensively documented positive theory of shopping behavior in the November 1986 Journal of Regional Science, and at Charles ReVelle, "The Maximum Capture or 'Sphere of Influence' Location Problem: Hotelling Revisited on a Network," which is an attempt to cope with the analytically intractible multi-firm, two-dimension location problem, in the May 1986 issue of the same Journal. Atsuyuki Okabe et. al. offer "A Statistical Analysis of the Spatial Association of Convenience-Goods Stores by use of a Random Clumping Model," in the February 1985 issue, and that paper cites some previous efforts to determine why some types of stores cluster and others scatter.