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joint-discovery-gatk4-local.wdl uses deprecated output syntax
In the GitHub-provided, official WDL files for running GATK, the "local" version of joint discovery uses a deprecated form of declaring outputs, which prevents it from being run as a subworkflow.
Note that another user already has a pull request with a fix for this issue. However, that has been pending for nearly a month now, so might be worth accepting: