I released abstract.rb 1.0.0.
http://rubyforge.org/projects/abstract/

'abstract.rb' is a library which enable you to define abstract method
in Ruby.

The followings are examples:

  ## example1. (shorter notation)
  require 'rubygems'   # if installed with 'gem install'
  require 'abstract'
  class Foo
    abstract_method 'arg1, arg2=""', :method1, :method2, :method3
  end

  ## example2. (RDoc friendly notation)
  require 'rubygems'   # if installed with 'gem install'
  require 'abstract'
  class Bar
    # ... method1 description ...
    def method1(arg1, arg2="")
      not_implemented
    end
    # ... method2 description ...
    def method2(arg1, arg2="")
      not_implemented
    end
  end

Abstract method makes your code more descriptive.
It is useful even for dynamic language such as Ruby.

--
regards,
kwatch