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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def subscribe(self, callback, existing=True):
 """Invoke `callback` for all distributions If `existing=True` (default), call on all existing ones, as well. """

        if callback in self.callbacks:
            return
        self.callbacks.append(callback)
        if not existing:
            return
        for dist in self:
            callback(dist)