How can I add a export var to override a nested section through @jinja syntax ??

How can I add a export var to override a field in nested section through @jinja syntax ??

There is my situation:

default_settings.toml:
```toml
[default]
base = "@jinja {{this.current_env | lower}}"
topic = "15021"

[default.mq]
routekey_prefix = "Topic-CAD-Dispatch-Cmd-RouteKey-"
routekey = "@jinja {{this.mq.routekey_prefix}}{{this.base}}.{{this.topic}}"
```

When I export this `topic=11`, this lazy loader can not substitute `{{this.topic}}` in the routekey correctly.I need some help,please.

Thank you.