revert to parent snapshot with vmrun

I am trying to script reverting to the current (parent) snapshot. 

Looks like the revertToSnapshot option REQUIRES a snapshot be named.  Since I am writing this script to work with hundreds of different VMs (whose snapshot names are not standard), I want the script to simply revert to the current parent.  I can use listSnapshots to see what the snapshot names are, but can not tell which is the current parent.

Anyone figured this out?

Thanks,

Pete