PREV function

["neo" <neo1@xxxxxxxxx>]

I have 
been using MS Pro for over 1 1/2 years and I do not understand the PREV 
function. It is not well explained in either the manual or the help file. How 
does it differ from Ref(X,-1)?
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I am 
trying to have a formula determine if I am long or short.
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Any 
help?
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<SPAN 
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