Force a given key/value pair into a certain form.
        
        Override the _conform_key and _conform_value if you want to change
        the mapping behaviour.

def _conform_pair(self, pair):
        """Force a given key/value pair into a certain form.
        
        Override the _conform_key and _conform_value if you want to change
        the mapping behaviour.
        
        """
        pair = tuple(pair)
        if len(pair) != 2:
            raise ValueError('MultiMap element must have length 2')
        return (self._conform_key(pair[0]), self._conform_value(pair[1]))