def METHOD_NAME(self, idx):
    """Select a subset of the observations in idata_orig.
    **Not implemented**: This method must be implemented on a model basis.
    It is documented here to show its format and call signature.
    Parameters
    ----------
    idx
        Indexes to separate from the rest of the observed data.
    Returns
    -------
    modified_observed_data : dict
        Dictionary containing both excluded and included data but properly divided
        in the different keys. Passed to ``data`` argument of ``model.sampling``.
    excluded_observed_data : str
        Variable name containing the pointwise log likelihood data of the excluded
        data. As PyStan cannot call C++ functions and log_likelihood__i is already
        calculated *during* the simultion, instead of the value on which to evaluate
        the likelihood, ``log_likelihood__i`` expects a string so it can extract the
        corresponding data from the InferenceData object.
    """
    raise NotImplementedError("sel_observations must be implemented on a model basis")