code: def sum_2_dictionaries(dicta, dictb):
      
      dict_out = dicta.copy()
      for key in dictb.keys():
          if 'key' in dict_out:
              # Add the sum for key in dictb to that of dict_out:
              dict_out[key] += dictb[key]
          else:
              # the key is not in the first dictionary - add it directly:
              dict_out[key] = dictb[key]
      return dict_out
docs: Given two dictionaries of totals, where each total refers to a key
         in the dictionary, add the totals.
         E.g.:  dicta = { 'a' : 3, 'b' : 1 }
                dictb = { 'a' : 1, 'c' : 5 }
                dicta + dictb = { 'a' : 4, 'b' : 1, 'c' : 5 }
         @param dicta: (dictionary)
         @param dictb: (dictionary)
         @return: (dictionary) - the sum of the 2 dictionaries