code: function(word, labels) {
      let links = this.links
      let text = word.text
      // If the word has tags, it is a parameter.
      if (word.tags.length > 0) {
        links = this.paramLinks
        text = word.tags[0]
      }
      let link
      if (links.has(text)) {
        link = links.get(text)
      } else {
        // We do not have a link to this.
        link = new Link(this.brain, this.getNode(word, labels))
        links.set(text, link)
      }
      labels.forEach(label => link.walkedBy(label))
      return link.target
    }
docs: eg. {word, tags} Returns the corresponding node.