delta(var[,n])

The two arguments are a variable and an integer n. The second argument may be omitted and then defaults to be 1. If n>0, the result is the value of the specified variable in row current+n minus the value in the current row; if n=0, the result is undefined; if n<0, the result is the value of the specified variable in the current row minus the value in row current+n; except that, if row current+n is beyond a cell with Break status, or beyond the end of the column, then the result is zero. See Transform Combination Restrictions.