Default fallback for global property, or "How to check if a val is defined globally?"

[Sailsman63]

Please forgive me if this turns out to be a very basic Scala/Ammonite question. I'm coming at it as a complete novice - actually, I'm evaluating mill as a build tool for a pure-java application ecosystem.

Is there a way to assign a default value to a variable if and only if that variable is not defined in ~/.mill/ammonite/predefScript.sc?

My use case is for a set of plugins to a full-stack application. Obviously, the plugins have a compile-time dependency on the core application. The application is not available as a maven/ivy/other dependency, and would not be well-suited to such deployment anyway. Therefore, the various plugins must treat it as an un-managed jar(s). I don't want to assume that all plugin developers have a full, standard dev checkout for the core application - they may have an actual working production installation - or that they even keep all of their development projects as siblings in the same folder. I also want the most likely case - siblings of a core development build - to just work out of the box.

In the absence of something like #1226, this is the only thing I've come up with that would allow a single configuration for multiple plugin builds.