class Class
  # Creates a class-variable attr_writer that can
  # be accessed both on an instance and class level.
  #
  #   require 'facet/class/attr_writer'
  #
  #   class MyClass
  #     cattr_writer :a
  #     def a
  #       @@a
  #     end
  #   end
  #   MyClass.a = 10
  #   mc = MyClass.new
  #   mc.a                #=> 10
  #
  def cattr_writer(*syms)
    syms.each do |sym|
      class_eval <<-EOS
        if ! defined? @@#{sym}
          @@#{sym} = nil
        end
        def self.#{sym}=(obj)
          @@#{sym} = obj
        end
        def #{sym}=(obj)
          self.class.#{sym}=(obj)
        end
      EOS
    end
  end
end