def split_by_indel_size(df, indel_class):
    """Split dataframe by indel size
    Args:
        df (pandas.DataFrame)
        indel_class (str): "s" for 1-nt, "m" for >1-nt indels
    Returns:
        df (pandas.DataFrame): only contains 1-nt 
                               or >1-nt indels
    """
    if indel_class == "s":
        return df[df["indel_size"] == 1].reset_index(drop=True)
    else:
        return df[df["indel_size"] > 1].reset_index(drop=True)