"I pasted your DOM demo code twice, and then I was able to access both window.x and window.x2 from within Nulan. Is this intentional?"
Yes, absolutely. It's how Nulan ensures that every variable is bound to a single "box". This also has the nice property that all the different "versions" of a variable can be accessed from the JavaScript side.
Ordinarily you can't access the different versions from within Nulan, but "window" is one of the ways you can do so. Another way would be the "&" macro which bypasses the Nulan compiler.
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"I also appended some DOM elements to the parent frame using "window.parent.document.append-child x", and they didn't get cleared on each update, but that's my own fault I guess. :-p"
Yeah, only the <iframe> is sandboxed. Of course you can refresh the page to clear it out. Don't worry, it'll save the text you typed in the textarea.