How to use other_bucket_key in ongr-io filters aggregation?

Question

I am using a filtersaggregation, and it all works in the kibana console. I need to translate this bit of code into ongr, but the documentation is sketchy. I can't find how to do "other_bucket_key".

"aggs": {
            "real_sale": {
              "filters": {
                "other_bucket_key": "genuine",
                "filters": {
                  "fake":{"match": {
                    "descriptions": "not_this"}
                  }
                }
              },
              

I wish I could just push it to the query before the aggregation, but an entry may have several nested thingies, where I must avoid "not_this".

So the question is: Is there documentation on other_bucket_key in ongr-io, or, better still, some examples?