How to specify varargs in a file task?

[motlin]

In a file task, I tried to make an arg accept multiple values with var=true.

#USAGE arg "<command>" var=true

This failed with the error:

usage: KdlError(KdlError { input: "arg \"<command>\" var=true", diagnostics: [KdlDiagnostic { input: "arg \"<command>\" var=true", span: SourceSpan { offset: SourceOffset(20), length: 4 }, message: Some("Expected identifier string"), label: Some("not identifier string"), help: None, severity: Error }] })