def penn_to_wordnet(tag):
    """
    Convert a Penn Treebank PoS tag to WordNet PoS tag.
    """
    if tag in ['NN', 'NNS', 'NNP', 'NNPS']:
        return 'n' #wordnet.NOUN
    elif tag in ['VB', 'VBD', 'VBG', 'VBN', 'VBP', 'VBZ']:
        return 'v' #wordnet.VERB
    elif tag in ['RB', 'RBR', 'RBS']:
        return 'r' #wordnet.ADV
    elif tag in ['JJ', 'JJR', 'JJS']:
        return 'a' #wordnet.ADJ
    return None