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