The Bath Tub Model
	0440949 Andreas van Cranenburgh

Abstract: A qualitative model of the water level in a bath tub 
	with faucet and drain.

The point of the model is to simulate the water level in the bath tub as
water pours from the faucet into the bath tub, while it leaves the bath tub
through the drain.

The faucet has three levels of water flow:
	- Zero (point)
	- Low (interval)
	- Medium (point)
	- High (interval)

The drain has two levels of water flow:
	- Plus (drain is open)
	- Zero (drain is closed)

The bath tub has 5 levels:
	- Lowest (point)
	- Low (interval)
	- Medium (point)
	- High (interval) 
	- Max (point)

What I wanted to add was that the water flow through the drain is equal to
medium water flow from the faucet, so that with low water flow the bath tub
stays empty, with high it would overflow, and with medium it would stay
constant. I have attempted to do this by adding a correspondence to
water flow "medium" and drain "plus" (open).

However, it is not working correctly yet. If the water flow is
medium then all possible states are generated, and if the water flow is low,
zero or high then no states are generated. For low and zero I would have
expected this, but not for high (bath tub should overflow).