anchor.diff:    }:
       runCommand "docker-layer-${name}" {
         inherit baseJson contents extraCommands;
  -      buildInputs = [ jshon rsync ];
  +      buildInputs = [ jshon rsync tarsum ];
       }
       ''
         mkdir layer
  
positive.code:     ln -sT ${docker.src}/components/engine/pkg/tarsum src/github.com/docker/docker/pkg/tarsum
      go build
  
      cp tarsum $out
    '';
  
    # buildEnv creates symlinks to dirs, which is hard to edit inside the overlay VM
  
negative.code: 
          # Compute the tar checksum and add it to the output json.
          echo "Computing checksum..."
          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
  
  