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Philipp F says that the KL won't help here. It just slows the algorithm down. It
does not help because the problem is not a degenerated one and our experience
shows that these type of problems are relatively well solved by MAP.

self._trafo \elem {1,2,3}. Therefore, either list needs to have
four items or should be accessed via [self._trafo - 1].
This way seem more readable to me.

Bug can be triggered by creating a FFTOperator `op` and calling
`print(op.inverse.adjoint)`.