function PreFacet$1(params) {
    Facet$1.call(this, params);
  }
Partitions pre-faceted data into tuple subflows.
@constructor
@param {object} params - The parameters for this operator.
@param {function(Dataflow, string): Operator} params.subflow - A function
  that generates a subflow of operators and returns its root operator.
@param {function(object): Array<object>} params.field - The field
  accessor for an array of subflow tuple objects.