You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: demo.rs
    path of file: ./repos/freya/crates/torin/examples
    
    the code of the file until where you have to start completion: use std::collections::HashMap;

use torin::prelude::*;

// Custom measurer, useful to measure certain elements such as text with other libraries
pub struct CustomMeasurer;

impl LayoutMeasurer<usize> for CustomMeasurer {
    fn measure(
        &mut self,
        _node_id: usize,
        _node: &Node,
        _parent_size: &Area,
        _available_parent_area: &Area,
    ) -> Option<Size2D> {
        None
    }
}

#[derive(Clone)]
struct DemoNode {
    parent: Option<usize>,
    children: Vec<usize>,
    
    
    pub fn add(&mut self, node_id: usize, parent: Option<usize>, children: Vec<usize>, node: Node) {
        // Get the parent's height in the DOM
        let parent_height = parent
            .map(|p| self.nodes.get(&p).unwrap().height)
            .unwrap_or(0);

        // Assign the node a height just below its parent
        let height = parent_height + 1;

        self.nodes.insert(
            node_id,
            DemoNode {
                parent,
                children,
                height,
                node,
            },
        );
    }