给定一篇文章，统计其中出现次数最多的单词。


"Once upon a midnight dreary, while I pondered, weak and weary, Over many a quaint and curious volume of forgotten lore, While I nodded, nearly napping, suddenly there came a tapping, As of someone gently rapping, rapping at my chamber door."
出现次数最多的单词是 "a"，共出现了 5 次。