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Yes. Say, for example, that you create a bunch of subnets and you aggregate all of them on a list. Then you want to pass it as a comma separated value list to CloudFormation.

I managed to find a way using reduce. Python example:

```...                                                                "SubnetIds": functools.reduce(lambda a, b: pulumi.Output.concat(a, ",", b), map(lambda a: a.id, subnets))
...```
