Using GEV in PyMC v4

I would like use the generalized extreme value distribution in PyMC v4. This distribution is available in PyMC3, but for v4 I would need to create a custom distribution and use pm.DensityDist.

I am struggling to work out how to create a custom distribution in PyMC4 and I can’t find any working example notebooks.

Please could either:
a) pm.GenExtreme() get ported into PyMC4
b) An example notebook be made that shows how to create and use custom distributions in PyMC4.

Many thanks,
Harry

It looks like that distribution has been moved to the pymc experimental package and that development is ongoing, including work on a notebook illustrating its use. If you need this functionality, I might chime in on one of those issues.