How2 refer to priornext row Fields! Expression in ReportViewer   

How can I refer to Prior or Next row in a ReportViewer Expression.
As in: 
=Iif( (Fields!BMP.Value + Fields!BMPOffset.Value) <> (Fields[-1]!BMP.Value + 
Fields[-1]!BMPOffset) , "Transparent" , "WhiteSmoke")

date: Fri, 9 Nov 2007 11:16:00 -0800   author:   CarlZ