class Constraint
  class << self
    def defaultable_attribute(attr)
      attr_reader(attr)
      attr_accessor("default_#{attr}")
      alias_method "default_#{attr}?", "default_#{attr}"
      define_method("#{attr}=") do |value|
        self.send("default_#{attr}=", false)
        instance_variable_set("@#{attr}", value)
      end
    end
  end
end