def expand_span(span, expand_length=2):
    """
    Args:
        span (list): [st, ed]
        expand_length (int): length to add on the two sides

    Returns:
        expanded_span (list): [max(0, st-expand_length), ed + expand_length]
            Only use the span for indexing, no need to worry the case where
            (ed + expand_length) >= max_length.
    """
    return [max(0, span[0] - expand_length), span[1] + expand_length]