endog, exog are everywhere in our code,
similar model in two implementations
https://github.com/statsmodels/statsmodels/pull/1398/files#diff-ea4d0eabae3845ac325dfdf11c3cbd2dR1118
https://github.com/statsmodels/statsmodels/blob/master/statsmodels/sandbox/panel/mixed.py#L107
guess which one I prefer to read