docs: function Node(brain, word, labels, root) {
    this.brain = brain
    // Map from label to list of tags.
    this.labelTags = new Map()
    if (word) {
      this.text = word.text
      if (labels) {
        labels.forEach(label => this.labelTags.set(label, word.tags))
      }
    } else {
      this.text = ''
    }
    this.root = root || this
    // Map from words to Links pointing to Nodes corresponding to those words.
    this.links = new Map()
    // Map from parameters to Links pointing to corresponding Nodes.
    this.paramLinks = new Map()
  }