def METHOD_NAME(self, X, y=None):
    """Fit transformer to X and y.
    private _fit containing the core logic, called from fit
    Parameters
    ----------
    X : nested pandas DataFrame of shape [n_instances, n_features]
        each cell of X must contain pandas.Series
        Data to fit transform to
    y : ignored argument for interface compatibility
        Additional data, e.g., labels for transformation
    Returns
    -------
    self : reference to self
    """
    if self.lower is None:
        n_instances, _ = X.shape
        arr = [X.iloc[i, :].values for i in range(n_instances)]
        self.lower_ = self._get_min_length(arr)
    else:
        self.lower_ = self.lower
    return self