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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)`.

Related to Issue #62

The reason for this is the following. MultiFields behave very similar to
dictionaries except for the fact that dictionaries are mutable and MultiFields
are immutable. Therefore, MultiField.update() needs to return a new object. That
is different to the dictionary.update() functionality and therefore unintuitive.

MultiField.union() can do the exact same thing as update() used to do.