shift(var,n)

The arguments are a variable and an integer. If the integer, n, is positive, the variable is shifted down n rows, and the first value is duplicated in the first n rows. If n is negative, the variable is shifted up |n| rows, and the last value is duplicated in the last |n| rows. In either case, there is no change to the length of the variable, the number of rows for which the variable has a value. See Transform Combination Restrictions.