docs: def lr_padding(self, terms):
          '''
          Pad doc from the left and right before adding,
          depending on what's in self.lpad and self.rpad
          If any of them is '', then don't pad there. 
          '''
          lpad = rpad = []
          if self.lpad:
              lpad = [self.lpad] * (self.n - 1) 
          if self.rpad:
              rpad = [self.rpad] * (self.n - 1) 
          return lpad + terms + rpad