enh: improve when-then-otherwise to include chaining #668
Description
We would like to learn about your use case. For example, if this feature is needed to adopt Narwhals in an open source project, could you please enter the link to it below?
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Please describe the purpose of the new feature or describe the problem to solve.
As discussed in #588, where a simple when-then-otherwise expression is added, the case for chaining when-then expressions is left to be done at some later point. This will complete and close #47
Suggest a solution if possible.
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If you have tried alternatives, please describe them below.
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Additional information that may help us understand your needs.
I would be willing to write this code, as looking to complete the open #588 and this is a natural extension
Initial draft proposal can be found at #669