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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 watch(self, selector, callback):
 """Call a function whenever a stream changes. Args: selector (DataStreamSelector):
 The selector to watch. If this is None, it is treated as a wildcard selector that matches every stream. callback (callable):
 The function to call when a new reading is pushed. Callback is called as: callback(stream, value) """


        if selector not in self._monitors:
            self._monitors[selector] = set()

        self._monitors[selector].add(callback)