[r-t] Extents of minor

[Chris Poole]

> Simple question: how many extents of minor are there?
>
> Obviously, there are 720! ways of arranging the rows, but the vast majority
> of these will include jump changes.
> The maths required to produce the reduced list of true extents are somewhat
> beyond me....
>
> Percy

Let's not stop at minor and be a bit more general... what is the number of
extents on stage n?  (there must be a graph theorist out there somewhere!)

Chris