Place the rabbit pieces in a large pot and cover with water. Bring to boil over high heat. Reduce heat to medium temperature and simmer covered until meat is fork-tender (1-1 1/2 hours). Drain the pieces on paper towels. On a piece of waxed paper, mix the dry mustard and a little salt and pepper with the flour. Roll each piece of meat in the flour mixture. Melt the butter and margarine in a large frying pan or skillet, over high heat. Brown the rabbit pieces on each side. Keep the browned pieces hot in a warm oven until ready to serve.