function Subflow(pulse, parent) {
    Operator.call(this, pulse);
    this.parent = parent;
    this.count = 0;
  }
Provides a bridge between a parent transform and a target subflow that
consumes only a subset of the tuples that pass through the parent.
@constructor
@param {Pulse} pulse - A pulse to use as the value of this operator.
@param {Transform} parent - The parent transform (typically a Facet instance).