THERE is little outward sign that Walmart’s “supercentre” in Rogers, Arkansas differs from any other of the giant retailer’s outlets. Walmart conquered America with such “big box” stores—vast concrete blocks in an ocean of parking spaces. Inside stretches aisle upon aisle of merchandise, from Patti LaBelle sweet-potato pies to Ruger rifles and ScentSationals Wax Warmers, shaped like owls. But this particular store, near Walmart’s headquarters in Bentonville, is different. It is where the world’s biggest retailer hones some of the new ideas which, it hopes, will help it keep its crown.