A while back, Dan Connolly and I argued this point. He said that XML documents were like integers: just because you write a new one down doesn't mean you've created anything new, and likewise writing something to a file doesn't cause the set of XML documents to grow either.
But I think an XML document is first and foremost a document, and documents can be created and destroyed. The sentence "John Rumpelmayer Jones sold me a goat, a bridge, a boat, a wedge, a coat, a dredge, a groat, a hedge, a stoat, a pledge, a moat, a midge, and a vote." is a grammatical English sentence, and a fortiori a document, that never existed before this posting.
If there is a poem beginning "In Xanadu did Kubla Khan" (and there is), then is there a poem whose text is solely:
Singing of Mount Abora
and every other possible poem-fragment?
The translation of this poem into German exists and is a poem (it begins "Im Xanadu ließ Kubla Khan"). Does the translation into Zarphatic (the dead Jewish language which is to French as Yiddish is to German and Ladino to Spanish) also exist and also constitute a poem, regardless of the fact that no one has ever made such a translation, nor will (very likely) in future? I think not.