[r-t] Proposed definition of a peal

[Iain Anderson]

> 1) Consider the set of rows and take the current stage to be 
> the maximum stage at which each row is rung.
> 2) If the number of rows in the set is less than or equal to 
> the extent at the current stage, the rows must be distinct.
> 3) Otherwise remove a true extent's worth of rows from the 
> set and re-apply the truth test on the remaining rows.  (If 
> you can't do this, it's false.)

That doesn't work if there is a part extent of minor as well.
What happens at the moment is that we allow multiple extents at a lower
stage to be treated as equivalent to an extent at a higher stage.  Once we
have removed as many extents (or equivalent) at the highest stage as
possible, whatever rows are left have to be true at the highest remaining
stage.