The Wikipedia page for Regular Tree Grammars notes that if $L_1$ and $L_2$ are regular tree languages, than $L_1 \setminus L_2$ is as well. However, it doesn't define this quotient operation for trees, and doesn't give a reference for this specific fact.
I'm wondering if someone can point me to a formal definition of the quotient operaiton on tree languages, and a proof that they are indeed closed under this operation.