def _words_iter(sentences, conll_format):
    """Iterator over words in sentences.
    
    Args:
        sentences: An iterator containing a lists of lists
            representing sentences (CWB format)

    Returns:
        An iterator

    """
    index = 0
    if conll_format:
        index = 1
    for sentence, sid in sentences:
        for token in sentence:
            yield token[index]
