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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 on(self, eventName, cb):
 """ Subscribe to an igv.js event. :param Name of the event. Currently only "locuschange" is supported. :type str :param cb - callback function taking a single argument. For the locuschange event this argument will contain a dictionary of the form {chr, start, end} :type function """

        self.eventHandlers[eventName] = cb
        return self._send({
            "id": self.igv_id,
            "command": "on",
            "eventName": eventName
        })