human
Solve the following python programming problem given the following code:
def _keys_match(attribute) : 
    '
    Create an invariant for a ``field`` holding a ``pmap``.

    The invariant enforced is that the keys of the ``pmap`` equal the value of
    a particular attribute of the corresponding values.

    :param str attribute: The name of the attribute of the ``pmap`` values
        which must equal the corresponding key.
    :return: A function suitable for use as a pyrsistent invariant.
    ' 
    def key_match_invariant(pmap) : 
        if (pmap is None) : 
            return (True, '') 
for (key, value) in pmap.items () : 
            if (key != getattr (value, attribute)) : 
                return (False, '{} is not correct key for {}'.format (key, value)) 
return (True, '') 
return <MASK> 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: attribute
assistant
#Here is a proper replacement for <MASK>.
def key_match_invariant(