Song hurried to join him, blinking up at the forest with her eyebrows raised. That was different.
"Okay... that's not my field, and it's not your library... unless this is supposed to tell us that the books are haunted by the ghosts of dead trees."
Despite the joke, she was trying hard to rationalize what was going on. Of the problems likely in this test, she would have expected a merging of the two dream worlds, but here was a third one instead, and none of her senses were telling her if it had spawned from only one of their subconscious minds, or both.
So, being the hands-on scientist that she was, she tried to change it.

The bookcases began to fall from the trees. But they were falling into the upside-down sky, against the rain.

The look on Song's face suggested that this wasn't quite what she'd been going for.