MDX Function and Operator Reference - Professional SQL Server Analysis Services 2005 with MDX - page 1086

Figure A-24: .Lag () and .Lead ().
member .LastChild Returns: member Standard
See definition for .FirstChild
LastPeriods ( index [, member ]) Returns: set Standard
This function returns the set of index periods from member back to the
member lagging by index -1 from member. This is almost equivalent to
{ member.LAG (index - 1) : member }.