[ppml] countdown proposal

...different codec, same chop ...

We are on track to burn through just short of 1/3 of the remaining space
this year. Given that we are on a compound growth curve rather than a flat
10 /8's per year, there is not enough time left to implement this proposal.
I understand Geoff's numbers show a longer timeframe, but 13, 15, 18 is what
it looks like at the moment, so to implement a 2 year stop the event would
have to happen early next year. That is not far enough down the road for
people to react.

Tony