def process_line(line, hist):
"""Adds the words in the line to the histogram.
Modifies hist.
line: string
hist: histogram (map from word to frequency)
"""
# replace hyphens with spaces before splitting
line = line.replace('-', ' ')
for word in line.split():
# remove punctuation and convert to lowercase
word = word.strip(string.punctuation + string.whitespace) # 单词的分割要找到其本质特点，其首尾必然是字母（中部可能出现标点，如isn't）
word = word.lower()
# update the histogram
hist[word] = hist.get(word, 0) + 1