levelDownstream(nodeA, nodeB) {
    if (this.levels[nodeB.id] === undefined) {
      // set initial level
      if (this.levels[nodeA.id] === undefined) {
        this.levels[nodeA.id] = 0;
      }
      // set level
      this.levels[nodeB.id] = this.levels[nodeA.id] + 1;
    }
  }
@param {Node} nodeA
@param {Node} nodeB