From: Thomas Wieczorek <wieczo.yo@...>
Date: 2008-02-28T22:23:27+09:00
Subject: Re: Abstract class

There is no keyword to create abstract classes, but you can force the
implementation of some methods if you need to.

class Foo
  def bar
     raise NotImplementedError
  end
end

You will only get an exception at runtime, when something tries to
call it. So you should override it in a subclass.

class Bar < Foo
  def bar
     "Foobar"
  end
end