Method f_down

datafusion/common/src/tree_node.rs:1906–1909

(&mut self, node: &'n Self::Node)

        type Node = TestTreeNode<T>;

        fn f_down(&mut self, node: &'n Self::Node) -> Result<TreeNodeRecursion> {
            self.visits.push(format!("f_down({})", node.data));
            (*self.f_down)(node)
        }

        fn f_up(&mut self, node: &'n Self::Node) -> Result<TreeNodeRecursion> {
            self.visits.push(format!("f_up({})", node.data));