First of all, thanks for making this - so far it seems like a very convenient way to write grammars/parsers!

I am currently using Java 11, which is the LTS version.
So now I have to either upgrade to Java 12 or downgrade this library to 2.2.0, but then I would lose ExpressionGroup.wrapper().

Hopefully it would not be too much work to set the release target to 11 instead of 12. It might be as simple as adding

<configuration>
    <release>11</release>
</configuration>

to the pom file.