Jh Van vuuren Stellenbosch Unit for Operations Research in Engineering   Department of Industrial Engineering Stellenbosch University Private Bag X1 Stellenbosch, Western Cape 7602 South Africa   vuuren@sun.ac.za   http://www.vuuren.co.za/           JEL Areas: E1 : General Aggregative Models: General H2 : Taxation, Subsidies, and Revenue: General