old_contents: export {
    Aggregate,
    Apply,
    AxisTicks,
    Bin,
    Collect,
    Compare,
    CountIndex,
    CountPattern,
    CrossFilter,
    DataJoin,
    Encode,
    Extent,
    Facet,
    Field,
    Filter,
    Fold,
    Force,
    Generate,
    GeoPath,
    GeoPoint,
    GeoShape,
    Graticule,
    Impute,
    LegendEntries,
    LinkPath,
    Lookup,
    MultiExtent,
    Nest,
    NoOp,
    Pack,
    Params,
    Partition,
    Pie,
    Projection,
    Range,
    Rank,
    Reflow,
    Relay,
    ResolveFilter,
    Sample,
    Scale,
    Sieve,
    Stack,
    Stratify,
    Subflow,
    Tree,
    TreeLinks,
    Treemap,
    TupleIndex,
    Values,
    Voronoi
  } from 'vega-dataflow';
  
  export {default as Bound} from './Bound';
  export {default as ChartLayout} from './ChartLayout';
  export {default as Mark} from './Mark';
  export {default as Render} from './Render';
  
new_contents: export {
    Aggregate,
    Apply,
    AxisTicks,
    Bin,
    Collect,
    Compare,
    CountIndex,
    CountPattern,
    CrossFilter,
    DataJoin,
    Encode,
    Extent,
    Facet,
    Field,
    Filter,
    Fold,
    Force,
    Generate,
    GeoPath,
    GeoPoint,
    GeoShape,
    Graticule,
    Impute,
    Key,
    LegendEntries,
    LinkPath,
    Lookup,
    MultiExtent,
    Nest,
    NoOp,
    Pack,
    Params,
    Partition,
    Pie,
    Projection,
    Range,
    Rank,
    Reflow,
    Relay,
    ResolveFilter,
    Sample,
    Scale,
    Sieve,
    Stack,
    Stratify,
    Subflow,
    Tree,
    TreeLinks,
    Treemap,
    TupleIndex,
    Values,
    Voronoi
  } from 'vega-dataflow';
  
  export {default as Bound} from './Bound';
  export {default as ChartLayout} from './ChartLayout';
  export {default as Mark} from './Mark';
  export {default as Render} from './Render';
  
subject: Add Key operator.