def split_by_indel_size(df):
    """ Sort 1-nt and >1-nt indels
    Args:
        df (pandas.DataFrame)
    Returns:
        df_mono (pandas.DataFrame): df for 1-nt indels
        df_non_mono (pandas.DataFrame): df for >1-nt indels
    """
    df_mono = df[df["indel_size"] == 1]
    df_non_mono = df[df["indel_size"] > 1]

    df_mono.reset_index(drop=True, inplace=True)
    df_non_mono.reset_index(drop=True, inplace=True)

    return df_mono, df_non_mono