This has the advantage that we
get rid of the concept of an average rate
on the total capital stock which is perhaps
somewhat less directly relevant, less well
measurable+less well defined.
The marginal rate is the immediately relevant concept.
So when the old formualtion said that at the 
height of the boom the profit rate or the stock
declines, the new formulation says that
the marginal rate of profit (that on the ?)
at the height of the boom will ? short
of the standard rate, on (to be shown !)