code: def sum(self, callback=None):
          
          if callback is None:
              return sum(self.items)
  
          callback = self._value_retriever(callback)
  
          return self.reduce(lambda result, item: (result or 0) + callback(item))
docs: Get the sum of the given values.
  
          :param callback: The callback
          :type callback: callable or string or None
  
          :rtype: mixed