Where is the Box component used in the demos?

Hi! First, I'd like to say thanks to @artf for building such a slick library. I just started using it in my project and it was super easy to get set up.

I'm building a React app using grapesjs with the grapesjs-preset-webpage plugin. I've been trying mimic building a webpage like the one in the demo, and I noticed that there are a lot of Box components being used. Naturally, I expected to find Box under the Blocks menu, but I couldn't locate it. To my surprise, there wasn't a Box in the demo menu either! So where is the Box element coming from and how can I add it to my Blocks menu?

Thanks in advance for the assistance!