## rule

#### 1

However, such situations are the exception rather than the rule.

There are few exceptions to this rule.

It follows that any itinerary that obeys these four rules corresponds to a point in $B$.

#### 2

The possibility $A= B$ is ruled out in the same way.
Its role is to rule out having two or more consecutive $P$-moves (on the grounds that they can be performed in one go).