def calculate_pct_references(df, pct_reference):
    """
    Not used yet.  Given a total number of reference sequences this function
    will divide the sseqids by the reference sequence
    count for a pct_reference.
    """
    reference_count = df[['tax_id']].drop_duplicates()
    reference_count = reference_count.join(pct_reference, on='tax_id')
    reference_count = reference_count['count'].sum()
    sseqid_count = float(len(df['sseqid'].drop_duplicates()))
    df['pct_reference'] = sseqid_count / reference_count
    return df
