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.

https://docs.pola.rs/api/python/dev/reference/expressions/api/polars.coalesce.html

This is a very common operation when joining tables or combining two columns into a single column.

Suggest a solution if possible.

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If you have tried alternatives, please describe them below.

Instead of coalesce you can re-implement this with when-then-otherwise but it can be tedious to handle a variadic number of parameters.

Additional information that may help us understand your needs.

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