In addition to providing runnable code playpens, mdBook optionally allows them to be editable. In order to enable editable code blocks, the following needs to be added to the book.toml:

[output.html.playpen]
editable = true

To make a specific block available for editing, the attribute editable needs to be added to it:

```rust,editable
fn main() {
    let number = 5;
    print!("{}", number);
}
```

The above will result in this editable playpen:

fn main() {
    let number = 5;
    print!("{}", number);
}

Note the new Undo Changes button in the editable playpens.

By default, the editor is the Ace editor, but, if desired, the functionality may be overriden by providing a different folder:

[output.html.playpen]
editable = true
editor = "/path/to/editor"

Note that for the editor changes to function correctly, the book.js inside of the theme folder will need to be overriden as it has some couplings with the default Ace editor.