code: def name(self):
      
      base_str = 'd{}{}_'.format(self.derivative_count if
                                 self.derivative_count > 1 else '', self.expr)
      for var, count in self.variable_count:
          base_str += 'd{}{}'.format(var,  count if count > 1 else '')
      return base_str
docs: Save name which can be used for alphabetic sorting and can be turned
      into a kwarg.