def notify_to(observers, *args)
      if block_given? && !args.empty?
        raise ArgumentError.new 'cannot give arguments and a block'
      end
      observers.each do |observer, function|
        args = yield if block_given?
        observer.send(function, *args)
      end
    end
def notify_to(observers, *args)
      if block_given? && !args.empty?
        raise ArgumentError.new 'cannot give arguments and a block'
      end
      observers.each do |observer, function|
        args = yield if block_given?
        observer.send(function, *args)
      end
    end
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