Challenge: Stemming

You are given some text in text variable. Your task is to tokenize this text, remove the stop words, and apply stemming on tokens. To do this:

1. Import Porter Stemmer.
2. Convert text to lowercase and save it in text_lower.
3. Tokenize the text_lower string and save the result in tokens.
4. Load English stop words, convert them to set, and save in stop_words.
5. Filter out the stop words using list comprehension and save the result in filtered_tokens.
6. Create a Porter Stemmer and save it in stemmer.
7. Stem the tokens using list comprehension and save the result in stemmed_tokens.