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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 _update_callbacks(self, plot):
 """ Iterates over all subplots and updates existing CustomJS callbacks with models that were replaced when compositing subplots into a CompositePlot and sets the plot id to match the root level bokeh model. """

        subplots = self.traverse(lambda x: x, [GenericElementPlot])
        merged_tools = {t: list(plot.select({'type': TOOL_TYPES[t]}))
                        for t in self._merged_tools}
        for subplot in subplots:
            for cb in subplot.callbacks:
                for c in cb.callbacks:
                    for tool, objs in merged_tools.items():
                        if tool in c.args and objs:
                            c.args[tool] = objs[0]
                    if self.top_level:
                        c.code = c.code.replace('PLACEHOLDER_PLOT_ID', self.id)