def iteritems(self, key=_absent):
        """
        Parity with dict.iteritems() except the optional <key> parameter has
        been added. If <key> is provided, only items with the provided key are
        iterated over. KeyError is raised if <key> is provided and not in the
        dictionary.

        Example:
          omd = omdict([(1,1), (1,11), (1,111), (2,2), (3,3)])
          omd.iteritems(1) -> (1,1) -> (1,11) -> (1,111)
          omd.iteritems() -> (1,1) -> (2,2) -> (3,3)

        Raises: KeyError if <key> is provided and not in the dictionary.
        Returns: An iterator over the items() of the dictionary, or only items
          with the key <key> if <key> is provided.
        """
        if key is not _absent:
            if key in self:
                items = [(node.key, node.value) for node in self._map[key]]
                return iter(items)
            raise KeyError(key)
        items = six.iteritems(self._map)
        return iter((key, nodes[0].value) for (key, nodes) in items)