Can't figure out how to make a ladder

Hi. I’m new with Inform 7, and any kind of programming in general. I vaguely remember reading something in the documentation about ladders, but I can’t find it again. Anyway, here’s the code I have so far:

The ladder is a door.  The ladder is an open door.  It is south of the Artificial Intelligence Core Chamber.  It is south of the Hub. 

The Hub is a room.  It is above the Artificial Intelligence Core Room.

Can someone help me?

EDIT: I just wanted to mention that I haven’t read all the way through the documentation yet, so I apologize if the answer to my problem appears somewhere later in it.

You might be thinking of chapter 3.5 in the recipe book and example 43 that discuss staircases, ladders and the such.

That did it. Thanks!