Is x ∨ y ∨ z a tautology? Clearly not. Setting the three propositions each to **false**, the formula is false. But now consider: Is **A**
**−** **has** **−** **0** **∨** **A**
**−** **has** **−** **1** **∨** **A**
**−** **has** **−** **2** a tautology? The answer here is " yes of course, ... well, as
long we're interpreting those propositions to refer to a WaterWorld board. " We'll capture this notion by listing a bunch of **domain axioms** for WaterWorld: formulas
which are true for all WaterWorld boards.

There are a myriad of domain axioms which express the rules of WaterWorld. Here are a few of them:

- A − has − 0 ⇒ B − safe ∧ G − safe
- A − has − 2 ⇒ B − unsafe ∧ G − unsafe
- ...

A more complete list is here (Section 6.5.2: The domain axioms). Whenever we deal with WaterWorld, we implicitly take all these domain axioms as given.

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