human
Solve the following python programming problem given the following code:
def undunder_keys(_dict) : 
    "Returns dict with the dunder keys converted back to nested dicts

    eg::

        >>> undunder_keys({'a': 'hello', 'b__c': 'world'})
        {'a': 'hello', 'b': {'c': 'world'}}

    :param _dict : (dict) flat dict
    :rtype       : (dict) nested dict

    " 
    def f(key, value) : 
        parts = key.split ('__') 
        return { 
            parts [0] : (value if (len (parts) == 1) else f (parts [1], value)), 
} 
result = { 
        
} 
    for r in [f (k, v) for (k, v) in _dict.items ()] : 
        rk = list (r.keys ()) [0] 
        if (rk not in result) : 
            result.update (r) 
else : 
            result [rk].update (v [rk]) 
return result 

    
    

    Fix the buggy line: result [rk].update (v [rk])
assistant
#Here is Python code to fix the buggy line: result [rk].

result [rk].update (r [rk])