map (callback) {
      const content = callback(this.content, 'content', this)
      return new ParenthesisNode(content)
    }
Create a new ParenthesisNode whose child is the result of calling
the provided callback function on the child of this node.
@param {function(child: Node, path: string, parent: Node) : Node} callback
@returns {ParenthesisNode} Returns a clone of the node