code: def add(self, other: SingleData, fill_value=0):
      # TODO: add and __add__ are a little confusing.
      # This could be a more general
      common_index = self.index | other.index
      common_index, _ = common_index.sort()
      tmp_data1 = self.reindex(common_index, fill_value)
      tmp_data2 = other.reindex(common_index, fill_value)
      return tmp_data1.fillna(fill_value) + tmp_data2.fillna(fill_value)
  
imports: from __future__ import annotations
  from typing import Dict, Tuple, Union, Callable, List
  import bisect
  import numpy
  import pandas
label_desc: Use image node_id 4 for calling the SingleData obj's underlying member method code with example usage: obj.add(other, fill_value) and returns: unknown