def map_wn_pos(pos):
    """
    Map a Penn Treebank POS tag to a WordNet style one
    """
    if pos in ['NN', 'NNS']:  # not NNP!
        return 'n'
    elif pos.startswith('JJ'):
        return 'a'
    elif pos.startswith('RB'):
        return 'r'
    elif pos.startswith('VB'):
        return 'v'
    else:
        return None