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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 bound_symbols(self):
 """Set of bound SymPy symbols in the expression"""

        if self._bound_symbols is None:
            res = set.union(
                set([]), # dummy arg (union fails without arguments)
                *[_bound_symbols(val) for val in self.kwargs.values()])
            res.update(
                set([]), # dummy arg (update fails without arguments)
                *[_bound_symbols(arg) for arg in self.args])
            self._bound_symbols = res
        return self._bound_symbols