Display a partial autocorrelation plot
--------------------------------------
by Sean Becketti, Federal Reserve Bank of Kansas City

      ^pac^ varname [^if^ exp] [^in^ range] [^, nlags(^#^) constant^ ]

A graph of the first ^nlag^ partial autocorrelations is displayed along with a
one-standard-error band.  By default, 20 partial autocorrelations are plotted.
The standard error is approximated by 

                                     1
                        -----------------------------
                         sqrt(number of observations)

The partial autocorrelations are estimated from a sequence of ^nlag^
regressions.  If ^noconstant^ is specified, the regressions will be estimated
without a constant.  Otherwise, a constant will be included in the
regressions.