anchor.diff:      ln -sT ${docker.src}/components/engine/pkg/tarsum src/github.com/docker/docker/pkg/tarsum
       go build
   
  -    cp tarsum $out
  +    mkdir -p $out/bin
  +
  +    cp tarsum $out/bin/
     '';
   
     # buildEnv creates symlinks to dirs, which is hard to edit inside the overlay VM
  
positive.code: 
          # 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
          # 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
  