ACM references

[JuliaLawall]

The box for references generated for final versions is not marked as required, but omitting it generates a warning. This warning is furthermore only visible to the chairs, but not to the authors. The chair may prefer that there are no warnings. This is in practice confusing to the authors, who wonder where they should have seen this warning and whether there are other warnings they have overlooked.

It would be helpful for the authors and the chairs to have a consistent view.