def add_regressor(data, regressor, varname=None): 
    
    """
    adds a regressor to a `pandas.DataFrame` of target (predictand) values 
    for use in fbprophet 
    Parameters
    ----------
    data : pandas.DataFrame 
        The pandas.DataFrame in the fbprophet format (see function `prepare_data` in this package)
    regressor : pandas.DataFrame 
        A pandas.DataFrame containing the extra-regressor
    varname : string 
        The name of the column in the `regressor` DataFrame to add to the `data` DataFrame
    Returns
    -------
    verif : pandas.DataFrame
        The original `data` DataFrame with the column containing the 
        extra regressor `varname`
    """

    data_with_regressors = data.copy()
    
    data_with_regressors.loc[:,varname] = regressor.loc[:,varname]
    
    return data_with_regressors