http://www.ne.jp/asahi/hishidama/home/tech/scala/actor.html
http://d.hatena.ne.jp/j5ik2o/20110218/1298021538
http://d.hatena.ne.jp/toming/20110914/1315952978
http://kreisel.fam.cx/webmaster/clog/img/www.ice.nuie.nagoya-u.ac.jp/~h003149b/lang/actor/actor.html