code: function Link(brain, target) {
    this.brain = brain
    this.brain.links.push(this)
    this.target = target
    // Number of examples that walk through this link, given the example's label.
    this.labelCount = new Map()
    // Same as above, normalized by the number of examples for a label.
    // Populated by Brain.prototype.buildWeights().
    this.labelWeight = new Map()
  }
docs: target: a Node.