ternary ? operator in grouping

Is your feature request related to a problem? Please describe. what to group on can be conditioned on a field value, and a ternary operator will facilitate that

Describe the solution you'd like all( group(flag ? ID+variant : ID) ...

Describe alternatives you've considered if there is a way to hash from string to numbers, one can use that, but I could not find a function like that in https://docs.vespa.ai/en/reference/querying/grouping-language.html#expressions

Additional context Use case is deduplicating results, but what to de-dup on varies with the data