There is a PDF file available at AMA website below.

http://www.modelaircraft.org/templates/ama/compreg.asp
If you download RC-aerobatic PDF, you will see the description of Cobra with/without rolls. There is a small diagram also.
Everyone who fly Pattern read this competition regulation.

Checking out the NSRCA website is also good since it has judging information including any changes to the AMA rules etc.
http://nsrca.org

choose judging & rules on the menu.

Hope this helps,

IP