Member creativity::BookMarket::buy (Reservation &res) override

this is simple: eventually it would probably be better to have a firm (perhaps created when the Book is created?) that accepts payments, mitigating this override.

Argument output isn't quite right because boost::p_a doesn't know about the utf8 strings being used, so alignment fails. Figure out a way to fix that, perhaps by using std::wstring.

Add settings plus colour setting arguments to a file (~/.creativity-settings.rc).

Member creativity::Reader::updateDemandBelief ()

Consider adding books for each period they survive (and past periods). In other words, if the book is bought when \(age=2\), also add data points for \(age=0\) and \(age=1\), and keep doing so into the future until the book leaves the market.