code: def _evaluate_bg(self, n, xdata, p=None):
          
          # by default, use the fit values, otherwise, use the guess values.
          if   p is None and self.results is not None: p = self.results[0]
          elif p is None and self.results is None:     p = self._pguess
  
          # return None if there is no background function
          if self.bg[n] is None: return None
  
          # assemble the arguments for the function
          args = (xdata,) + tuple(p)
  
          # evaluate the function
          return self.bg[n](*args)
docs: Evaluates a single background function n for arbitrary xdata and p tuple.
  
          p=None means use the fit results