[amibroker] Please help with an exploration code

[possum51au]

I am trying to write an exploration which returns the closing price one year 
ago.  I know it will be simple but I seem to have a mental block.
Ideally I would like it to calculate the difference between today's close and 
that 1 year previous close, and present it as a percentage gain or loss.
Thanks in advance for any help
Raine