% KB3 from Learn Prolog Now! (http://www.learnprolognow.org/).

happy(vincent). 
listens2Music(butch). 
playsAirGuitar(vincent):- 
      listens2Music(vincent), 
      happy(vincent). 
playsAirGuitar(butch):- 
      happy(butch). 
playsAirGuitar(butch):- 
      listens2Music(butch).