function Facet$1(params) {
    Transform.call(this, {}, params);
    this._keys = fastmap(); // cache previously calculated key values

    // keep track of active subflows, use as targets array for listeners
    // this allows us to limit propagation to only updated subflows
    const a = this._targets = [];
    a.active = 0;
    a.forEach = f => {
      for (let i = 0, n = a.active; i < n; ++i) {
        f(a[i], i, a);
      }
    };
  }
Facets a dataflow into a set of subflows based on a key.
@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): *} params.key - The key field to facet by.