def lemmatize(tokens):

    """
    This function conducts lemmatization on a list of tokens (from the 'remove_stopwords' function).
    This shortens each word down to its root form to improve modeling results.
    """

    #initalize lemmatizer and lemmatize
    lemmatizer = nltk.WordNetLemmatizer()
    lemmatized_tokens = [lemmatizer.lemmatize(token) for token in tokens]

    return(lemmatized_tokens)