code: def pop(self, i=None):
          
          if i == None:
              i = len(self) - 1
  
          if i >= len(self._terms) or i < 0:
              raise ValueError('requested pop {}th term, but found only {} terms'\
                              .format(i, len(self._terms)))
  
          term = self._terms[i]
          self._terms = self._terms[:i] + self._terms[i+1:]
          return term
docs: remove the ith term from the term list
  
          Parameters
          ---------
          i : int, optional
              term to remove from term list
  
              by default the last term is popped.
  
          Returns
          -------
          term : Term