That was introduced by @josef-pkt in 84735c3f89. It seems to indeed be used, see https://github.com/statsmodels/statsmodels/blob/master/statsmodels/sandbox/distributions/extras.py#L558. Looks dodgy though. 

Josef can you comment on the status of ``Transf_gen`` and whether that should keep working in its current state? It looks like it's abusing the distribution framework.