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

        Returns: An iterator over the values of every item in the dictionary.

def iterallvalues(self, key=_absent):
        '''
        Example:
          omd = omdict([(1,1), (1,11), (1,111), (2,2), (3,3)])
          omd.iterallvalues() == 1 -> 11 -> 111 -> 2 -> 3

        Returns: An iterator over the values of every item in the dictionary.
        '''
        if key is not _absent:
            if key in self:
                return iter(self.getlist(key))
            raise KeyError(key)
        return self._items.itervalues()