> How can I make sure a package is always up to date?

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  bundle agent main
  {
    vars:
        "latest_pkgs" slist => { "singularity", "singularity-devel" };

    packages:

      "$(latest_pkgs)"
        policy => "present",
        comment => "Important to automatically track the latest and greatest because ....",
        version => "latest";
  }

The packages promise implementation shown above has been available since 3.7.0.