Suppose we wanted to write rules for a character who will interact with doors in other locations even when the player is not present. This poses a little challenge: doors are actually single objects, and -- with the same shuffling of stage properties that applies to backdrops -- they are moved as needed to represent the door object in whatever room contains the player.
because, even if Bernard's location is connected by doors to other places, the actual representation of that door may not be "in" Bernard's location, from the model's point of view, at this exact moment.
This does not, of course, mean that we can't ask this question; just that we have to be a little cleverer about how we phrase it. Every door has properties that correspond to the two locations
the front side of the blue door (a room, which is arbitrarily one side of the door)
the back side of the blue door (arbitrarily the other side)
We can make this information easier to check with a conditional relation, like so:
Liminality relates a door (called X) to a room (called Y) when the front side of X is Y or the back side of X is Y. The verb to be a threshold of means the liminality relation.
And this allows us to write rules that have characters interacting with doors even in the player's absence:
{*}"Wainwright Acts"
The Waiting Room is a room. The waiting room door is west of the Waiting Room and east of the Gents' Loo. The Waiting Room door is an open door. "The waiting room door [if open]stands open[otherwise]is shut firmly[end if]."
Liminality relates a door (called X) to a room (called Y) when the front side of X is Y or the back side of X is Y. The verb to be a threshold of means the liminality relation.