This example demonstrates a PJAX-style navigation interface where clicking links updates the page content without a full reload, using the browser’s History API to manage the back and forward buttons. The code defines a `pjax` function that attaches click handlers to all `ul li a` links, calls `history.pushState` with the target URL, and loads the new page fragment via `d3.html`. It replaces the content of `#main` with the corresponding element from the fetched document and re-applies the link handlers. A `popstate` listener loads the previous state when the user navigates backward or forward.