STELLARTON, N.S. — Sobeys parent Empire Co. has reached a deal to acquire the grocery and gas assets of Co-Op Atlantic. The Co-op has about 170 supermarkets and convenience stores under four different banners. Only about 10 stores would actually change hands under the deal, and independent dealers with have to make their own arrangements with Sobeys. But the deal is already worrying independent grocers, who see in it the threat of further concentration of the market in the hands of a few large operators. By making Sobeys a bigger supplier in the Maritimes, the deal could give it more sway in setting prices and determining inventory.