refinement.rb

class Base
  A_VALUE = 2
  def foo
  end
end

module Refiner
  refine Base do
    def foo
      old_value = super
      if old_value % 2 != 0
        raise "NOT EVEN"
      end
    end
  end
end

class Sub < Base
  def foo
    super
    return A_VALUE
  end
end

class BadSub < Base
  def foo
    super
    return A_VALUE + 1
  end
end

using Refiner


Sub.new.foo
BadSub.new.foo