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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 bind(self, callback):
 """Bind a ``callback`` to this event. """

        handlers = self._handlers
        if self._self is None:
            raise RuntimeError('%s already fired, cannot add callbacks' % self)
        if handlers is None:
            handlers = []
            self._handlers = handlers
        handlers.append(callback)