function Nest(params) {
    Transform.call(this, null, params);
  }
Nest tuples into a tree structure, grouped by key values.
@constructor
@param {object} params - The parameters for this operator.
@param {Array<function(object): *>} params.keys - The key fields to nest by, in order.
@param {boolean} [params.generate=false] - A boolean flag indicating if
  non-leaf nodes generated by this transform should be included in the
  output. The default (false) includes only the input data (leaf nodes)
  in the data stream.