def extract_sentences(lines):
    """
    Extracts the non-empty sentences and their numbers of the "lines" field in
    a JSON object from a FEVER wiki-pages JSONL file.
    """
    sentences = []

    sentence_index = 0
    for line in lines.split('\n'):
        tokens = line.split('\t')
        if not tokens[0].isnumeric() or int(tokens[0]) != sentence_index:
            # skip non-sentences, caused by unexpected \n's
            continue
        else:
            sentences.append((tokens[1], tokens[0]))
            sentence_index += 1

    return sentences