anchor.diff:      baseJson,
       # Files to add to the layer.
       contents ? null,
  +    # When copying the contents into the image, preserve symlinks to
  +    # directories (see `rsync -K`).  Otherwise, transform those symlinks
  +    # into directories.
  +    keepContentsDirlinks ? false,
       # Additional commands to run on the layer before it is tar'd up.
       extraCommands ? ""
     }:
  
positive.code:     runAsRoot,
      # Files to add to the layer. If null, an empty layer will be created.
      contents ? null,
      # JSON containing configuration and metadata for this layer.
      baseJson,
      # Existing image onto which to append the new layer.
  
negative.code:     fromImageTag ? null,
      # Files to put on the image (a nix store path or list of paths).
      contents ? null,
      # When copying the contents into the image, preserve symlinks to
      # directories (see `rsync -K`).  Otherwise, transform those symlinks
      # into directories.
      keepContentsDirlinks ? false,
      # Docker config; e.g. what command to run on the container.
      config ? null,
      # Optional bash script to run on the files prior to fixturizing the layer.
  