anchor.diff:      # How much disk to allocate for the temporary virtual machine.
       diskSize ? 1024,
       # Commands (bash) to run on the layer; these do not require sudo.
  -    extraCommands ? "", uid ? 0, gid ? 0
  +    extraCommands ? ""
     }:
       # Generate an executable script from the `runAsRoot` text.
       let runAsRootScript = shellScript "run-as-root.sh" runAsRoot;
  
positive.code:           echo "Adding $item..."
            rsync -ak --chown=0:0 $item/ layer/
          done
        '';
  
        postMount = ''
  
negative.code:         echo "No contents to add to layer."
        fi
  
        chmod ug+w layer
  
        if [[ -n $extraCommands ]]; then
          (cd layer; eval "$extraCommands")
        fi
  