Reverse the order of all items in the dictionary.

        Example:
          omd = omdict([(1,1), (1,11), (1,111), (2,2), (3,3)])
          omd.reverse()
          omd.allitems() == [(3,3), (2,2), (1,111), (1,11), (1,1)]

        Returns: <self>.

def reverse(self):
        """
        Reverse the order of all items in the dictionary.

        Example:
          omd = omdict([(1,1), (1,11), (1,111), (2,2), (3,3)])
          omd.reverse()
          omd.allitems() == [(3,3), (2,2), (1,111), (1,11), (1,1)]

        Returns: <self>.
        """
        for key in six.iterkeys(self._map):
            self._map[key].reverse()
        self._items.reverse()
        return self