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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 penn_to_wn(tag):
 """ Convert between a Penn Treebank tag to a simplified Wordnet tag """

    if tag.startswith('N'):
        return 'n'

    if tag.startswith('V'):
        return 'v'

    if tag.startswith('J'):
        return 'a'

    if tag.startswith('R'):
        return 'r'

    return None