* node in the tree (or from the startNode). If the function returns true, the node is kept
* otherwise it is filtered. If a node is filtered, its children are also filtered.
* @param {Function} fn The filter function
* node in the tree (or from the startNode). If the function returns true, the node is kept
* otherwise it is filtered. If a node is filtered, its children are also filtered.
* @param {Function} fn The filter function