Reinheitsgebot

Congratulations, you have found an easter egg: German law mandates that only certain ingredients can go into beer. Similarly, PureDarwin purity law mandates that only open source frameworks and libraries may be used. 

The Reinheitsgebot is the German law that regulates wich ingredients can go into beer: water, barley, hops and malt. (Only recently has it been updated by the Provisional German Beer law, which also allows wheat an cane sugar). For more information on the Reinheitsgebot, see Wikipedia: German purity law.