This example shows a collapsible rectangle tree visualization of the Flare software package hierarchy, where nodes are rendered as small rectangles with labels and colored strokes indicating whether they have children. Clicking a node expands or collapses its subtree with smooth animated transitions. The code uses the D3.js v6 library with `d3.tree()`, `d3.linkHorizontal()`, and `d3.zoom()`, rendering SVG elements with data from the `flare-2.json` file.