X{T-K}
X [ tseries ] - Tseries object whose time-recursive lag will be created.
T [ trec ] - Initialized trec object.
K [ numeric ] - Integer scalar specifying the lag.
The tseries object, X, referenced by T-K in a time-recursive expression will, in each iteration, return a value that corresponds to period t-K, where t is the currently processed date from the vector of dates (or date range) associated with the trec object, T.