*
* Example 1.5.3 from page 30
*
open data uspop.dat
calendar(years=10) 1790
data(format=free,org=columns) 1790:1 1990:1 uspop
*
* To get a series differenced more than one, you can use the DIFFS option to
* indicate the number of differencings needed.
*
diff(diffs=2) uspop / d2pop
graph(footer="Figure 1-23 Twice-Differenced U.S. Population")
# d2pop