docs: def subtract_by_key(dict_a, dict_b):
      """given two dicts, a and b, this function returns c = a - b, where
      a - b is defined as the key difference between a and b.
  
      e.g.,
      {1:None, 2:3, 3:"yellow", 4:True} - {2:4, 1:"green"} =
          {3:"yellow", 4:True}
  
      """
      difference_dict = {}
      for key in dict_a:
          if key not in dict_b:
              difference_dict[key] = dict_a[key]
  
      return difference_dict