device mapper help

[William Thompson <wt@electro-mechanical.com>]

I'm not on the list.  Please always keep me in the CC.

Is there a tool somewhere that will allow me to merge a snapshot into the
parent device?

For instance, if I have a snapshot of /dev/sdb like this:
sdb-snap1: 0 16777216 snapshot /dev/sdb /dev/sdb-snap1-backing P 8
sdb-snap2: 0 16777216 snapshot /dev/sdb-snap1 /dev/sdb-snap2-backing P 8
...

Lets say that I want to update sdb with the data in /dev/sdb-snap1-backing. 
What would I do to do this?  Is there some tool that does this?  I
understand that there can be problems with snapshots of snapshots like this,
but that is not the question.