anchor.diff:  
           # Compute the tar checksum and add it to the output json.
           echo "Computing checksum..."
  -        ts=$(${tarsum} < $out/layer.tar)
  -        cat ${baseJson} | jshon -s "$ts" -i checksum > $out/json
  +        tarhash=$(${tarsum}/bin/tarsum < $out/layer.tar)
  +        cat ${baseJson} | jshon -s "$tarhash" -i checksum > $out/json
           # Indicate to docker that we're using schema version 1.0.
           echo -n "1.0" > $out/VERSION
   
  
positive.code: 
        # Compute a checksum of the tarball.
        echo "Computing layer checksum..."
        tarsum=$(${tarsum} < $out/layer.tar)
  
        # Add a 'checksum' field to the JSON, with the value set to the
        # checksum of the tarball.
        cat ${baseJson} | jshon -s "$tarsum" -i checksum > $out/json
  
        # Indicate to docker that we're using schema version 1.0.
        echo -n "1.0" > $out/VERSION
  
negative.code:   }:
      runCommand "docker-layer-${name}" {
        inherit baseJson contents extraCommands;
        buildInputs = [ jshon rsync tarsum ];
      }
      ''
        mkdir layer
  