function _traverse(node, callback) {
      node.forEach(function (child, path, parent) {
        callback(child, path, parent);

        _traverse(child, callback);
      });
    }
Recursively traverse all nodes in a node tree. Executes given callback for
this node and each of its child nodes.
@param {function(node: Node, path: string, parent: Node)} callback
         A callback called for every node in the node tree.