A classic way of introducing pollution regulation into an urban area is the "rollback" method. First you determine the maximum tolerable level of a particular pollutant, \(x_{\mathrm{s}}\). Next you find the maximum current concentration of that pollutant in the urban area, \(x_{\mathrm{m}}\). Presumably \(x_{\mathrm{m}}>x_{\mathrm{s}}\) (otherwise, there is no problem). Regulations then require all polluters in the city to reduce emissions from their current level of \(e_{\mathrm{m}}\) to \(e_{\mathrm{s}}=e_{\mathrm{m}}\left(x_{\mathrm{s}} / x_{\mathrm{m}}\right)\). Explain how this would work to bring pollution levels down to an acceptable point. What might be the efficiency problems with such an approach?