The script defines a function run() with 5 parameters: a link to webpage and four words w1, w2, w3, w4.
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f1, f2, f3, f4 are the frequencies of w1,w2,w3,w4 in the webpage.
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The run() function returns a set of all the words in the webpage that have a higher frequency than the lowest value among f1,f2,f3,f4 and a lower frequency than the highest value among f1,f2,f3,f4.
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If one of the 4 words does not appear in the webpage, it assumes that its frequency is equal to zero.