这一行为也适用于词汇表以外的单词，如果将它们以小写字母的形式输入到字典中的话，例如ipod。
This behavior applies also to out-of-vocabulary words if you type them in lower-case letters into your dictionary, such as ipod.
写一个读取文件的程序，把每一行拆分成一个个词，去掉空白和标点符号，然后把所有单词都转换成小写字母的。
Write a program that reads a file, breaks each line into words, strips whitespace and punctuation from the words, and converts them to lowercase.
中间线：在一行英文字里，沿着小写字母上端的一条假想线条。
Mean line: Imaginary line which runs along the top of the lower-case letters in a line of text.
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