def excess(topic_srs, ref_srs):
    """Calculates the excess of a topic series vs. a reference series.

    Excess is simply the difference between the topic series and a reference 
    series. If passing returns, log returns should be used since the function 
    simply subtracts to calculate the difference.

    Args:
        topic_srs (Pandas DataFrame of float): The topic series.
        market_srs (Pandas DataFrame of float): The reference series.
    
    Returns:
        Pandas DataFrame of float: The difference between the topic series and 
        the reference series.
    """
    return topic_srs - ref_srs