Is there general rules for predict the structure of the result data tree?

It’s a question that’s been bothering me for a long time…
I think I DO know how two data tree match, but I cannot find a general rule to predict how the structure of the result tree look like, especially when the two matching tree are a little bit strange.
Just as the picture below, the two imput trees are a little bit unnormal, and I cannot find any rules to
predict the result tree‘s structure. I know that in such a situation, maybe it’s meaningless to ask for a general rule,but I am keenly awaited that someone can tell me that it really exists.