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 $B$.
[see also: exclude, preclude]
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).
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