Enforce interface manually because sometimes ruby lets you down.

interface.rb

class Coupon

  class << self
    def enforce_interface!
      methods = self.instance_methods(false)
      [:m].each do |method_name|
        unless methods.include? method_name
          raise "#{method_name} should be implemented"
        end
      end
    end
  end

end
 
class SharingCoupon < Coupon

  def m
  end

  enforce_interface!
end