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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def filter_stopwords(phrase):
 """Filter out stop words and return as a list of words"""

    if not isinstance(phrase, list):
        phrase = phrase.split()

    stopwords = ['the', 'a', 'in', 'to']
    return [word.lower() for word in phrase if word.lower() not in stopwords]