Overriding list entry in Hydra, from another YAML

In Hydra config YAML files, I have the following structure in the file inner.yaml:

key_a:    - entry_a_1: xxxx      entry_a_2: xxxxx    - entry_a_3: xxxx      entry_a_4: xxxxx

What I want is to be able to override the entry_a_M from another YAML file, for example at the file outer.yaml:

defaults:    - inner_config@outer_inner_config: inneroutter_inner_config:    entry_a_1: YYYY

I have tried with key_a.0.entry_a_1: WWWW and other combinations but it doesn't work.

Please note:

• I don't want to override it from the CLI
• If there are keys in each list item, e.g. - key: [entry_a_1...] then it can be done, as shown in question here. But it is not my case and it would not work with having that key in the list entry, in my case.

Any answers on that?