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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 _style(self, retval):
        "Applies custom option tree to values return by the callback."
        if self.id not in Store.custom_options():
            return retval
        spec = StoreOptions.tree_to_dict(Store.custom_options()[self.id])
        return retval.opts(spec)