How to use the vega-dataflow.Tuple.ingest function in vega-dataflow

}),
      prev = this._internal || [], curr = [], i, n;

  function level(index, node, values) {
    var vals = aggrs[index].execute(values);

    node[childField] = vals;
    vals.forEach(function(n) {
      n[parentField] = node;
      curr.push(Tuple.ingest(n));
      if (index+1 < fields.length) level(index+1, n, n[childField]);
      else n[childField].forEach(function(c) { c[parentField] = n; });
    });
  }

  var root = Tuple.ingest({});
  root[parentField] = null;
  curr.push(root);
  level(0, root, data);

  // update changeset with internal nodes
  for (i=0, n=curr.length; i

vals.forEach(function(n) {
      n[parentField] = node;
      curr.push(Tuple.ingest(n));
      if (index+1 < fields.length) level(index+1, n, n[childField]);
      else n[childField].forEach(function(c) { c[parentField] = n; });
    });
  }

function tuple(gb, gv, ob, ov) {
  var t = {_imputed: true}, i;
  for (i=0; i