Initial version, with average method.
First try.
Works a bit.
One version of median.
Ok, aggregation by median is in.
Now written in terms of variance; correct??
Ok, this works fairly well.
Only 5 reviews, on average.
Ok, this works very well.
Even more realistic numbers.
Ok, this shows the merit.
Can do plots.  Bias works well; variance less so.
Removed unnecessary code.
Ok, better inference of variance.
Checkpoint.
Fixed error.
Checkpoint.
Fixed a serious bug in the propagation from users to items.
Ok, this works quite well.
This works, very well.