code: def count(self, v):
          
          ctr = 0
          for item in reversed(self):
              if item == v:
                  ctr += 1
          return ctr
docs: Count occurrences of value v in the entire history. Note that the subclass must implement the __reversed__
          method, otherwise an exception will be thrown.
          :param object v: The value to look for
          :return: The number of occurrences
          :rtype: int