This example demonstrates a collapsible list that expands and collapses nested tree data on click. It uses d3’s `select`, `selectAll`, `data`, `enter`, and `append` methods to bind an array of objects (with “Letter” and “Number” parent nodes) to HTML `li` elements inside a `ul`. Clicking a parent toggles its child items by adding or removing a nested `ul` via event handlers `expand` and `collapse`. The visualization relies on D3’s data join to manage the dynamic list structure.