explicitely. Otherwise -W2 option makes following warning.
  "warning: instance variable @head not initialized".
  This issue was founded by Andrew Grimm. Thanks Andrew!
  [ruby-core:35435][Bug#4471]



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@31032 b2dd03c8-39d4-4d8f-98ff-823fe69b080e