[ppml] geo addressing

[Tony Li]

> By selecting cities larger than 100,000 to be centers of aggregation
> we allow for areas which could see considerable population growth.
> Such growth tends to add to existing cities rather than create new
> ones. I do not suggest that city address allocation size should be
> based on current population. I would like to see it based on future
> population as determined by economists, statisticians, geographers
> who understand the limits to growth in order to make realistic
> projections.


It seems like getting this exactly right is not so important.  If you  
overshoot,
then you have a prefix that is underpopulated, but you still aggregate
everyone in the metro.  If you undershoot, you need a second prefix, but
you still get ample aggregation from the first prefix.

As has been observed before most of the benefits of aggregation come
from aggregating the leaves of the addressing tree.

Tony