def define_cmp_method
      keys = @keys
      define_method(:cmp?) do |comparator, other|
        keys.all? { |key| send(key).send(comparator, other.send(key)) }
      end
    end
def define_cmp_method
      keys = @keys
      define_method(:cmp?) do |comparator, other|
        keys.all? { |key| send(key).send(comparator, other.send(key)) }
      end
    end
Define an #cmp? method based on the instance's values identified by #keys

 @return [undefined]

 @api private