def offset_fragments(fragments, offset):
    """Correct indices of fragments with an offset.

    When an article is stripped of its beginning, all fragments need to be reindexed
    with the length of what was stripped.
    """
    res = []
    for f in fragments:
        assert f.start >= offset, f'{str(f)} >= {offset}, fragments = {str(fragments)}'
        res.append(f - offset)  # Note: substraction from the Fragment subtracts from its start and end indexes
    return res