code: function (node, result, cache) {
      // node.left/right {}
      return this.gen(
        Random.boolean() ? node.left : node.right,
        result,
        cache
      )
    }
docs: /*
  {
  type: 'alternate',
  offset: 0,
  text: '',
  left: {
  boyd: []
  },
  right: {
  boyd: []
  }
  }