docs: def minmax_candidates(self):
          '''Get points where derivative is zero.
  
          Useful for computing the extrema of the polynomial over an interval if
          the polynomial has real roots. In this case, the maximum is attained
          for one of the interval endpoints or a point from the result of this
          function that is contained in the interval.
          '''
          from numpy.polynomial import Polynomial as P
          p = P.fromroots(self.roots)
          return p.deriv(1).roots()