>> help adf
  PURPOSE: carry out DF tests on a time-series vector
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  USAGE: results = adf(x,p,nlag)
  where:      x = a time-series vector
              p = order of time polynomial in the null-hypothesis
                  p = -1, no deterministic part
                  p =  0, for constant term
                  p =  1, for constant plus time-trend
                  p >  1, for higher order polynomial
          nlags = # of lagged changes of x included      
这段help看得不怎么懂，请问p的4种情况分别是什么。据我所知，adf检验只要检验3个式子。他怎么有4个？