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kotlinlang

in my `application.conf`  i have
```port = ${PORT}```
this works fine on heroku and when this is set in the environment, but can i somehow set eg. `8080`  as fallback ?
could not find how to do that in the HOCON spec

it's a two line thing:
```port = 8080
port = ${?PORT}```

but that seemed to not work.. it still crashes:
```Exception in thread "main" com.typesafe.config.ConfigException$UnresolvedSubstitution: application.conf @ file:/home/nikky/dev/pentagame/backend/build/resources/main/application.conf: 4: Could not resolve substitution to a value: ${PORT}```


<https://github.com/lightbend/config#optional-system-or-env-variable-overrides>