1class InputCell:
 2    def __init__(self, initial_value):
 3        self.value = None
 4
 5
 6class ComputeCell:
 7    def __init__(self, inputs, compute_function):
 8        self.value = None
 9
10    def add_callback(self, callback):
11        pass
12
13    def remove_callback(self, callback):
14        pass