human
Solve the following python programming problem given the following code:
def test_FitMutator(self) : 
    def mutate(Xb, **_) : 
        self.count += 1 
        Xb -= 0.5 
self.nn.callback = { 
        'on_batch_start' : t, 
} 
    for t in SPARSE_TYPES : 
        sparse_matrix = getattr (scipy.sparse, t) 
        X = sparse_matrix ((8, 4), dtype = numpy.float32) 
        y = numpy.zeros ((8, 4), dtype = numpy.float32) 
        self.count = 0 
        assert_equal (0, self.count) 
        self.nn._fit (X, y) 
        assert_equal (8, self.count) 

    
    

    Fix the buggy line: 'on_batch_start' : t ,
assistant
#Here is Python code to fix the buggy line: 'on_batch_start' : t ,
.

'on_batch_start' : def mutate(,