Mercedes is working to complete the circle, combining the frugality of its smallest A- and B-Class offerings with the luxury of its S-Class offerings. For the U.S. the S400 Hybrid began the earnest trek down that road, followed by the announcement of the four-cylinder S-Class, and at Frankfurt, Mercedes has introduced the S500 Vision Plug-in Hybrid concept. The home of the pointed star sees it as putting "the luxury class on the road to the three-liter...