19:2919:29, 29 March 2011diffhist+2,880
Externality
Added a positive externality that is out of the norm that attempts to walk the individual through the process of externalities.
21:2021:20, 9 March 2011diffhist+1,656
Substitute good
Added sources and a couple introductory sentenes. Added sentences in the examples section to distinguish between consumer and producer substitution