How to handle sale of partial lots of stock?

[John Schoffstall]

Greetings! Running 1.8.10 on Debian etch. New GnuCash user, new list 
user, new Linux user.

I've been climbing the GnuCash learning curve for the past week, been 
happy with the software, and making good progress until now. I've hit a 
snag I can't get past, though.

How do you sell a partial lot of stock -- e.g., selling only 100 of 200 
owned shares -- without unbalancing your balance sheet? I've tried 
creating two lots, each of 100 shares and only selling one, but that 
still whacks the balance sheet.

The examples in the docs all seem to show only the purchase and sale of 
equivalent amounts. I've googled the net and the mailing list archives, 
but can't find an answer I can understand.

This is a common situation; there must be an answer. Can anyone help?

== John