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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 Sympy symbols that are eliminated by evaluation."""

        if self._bound_symbols is None:
            res = set()
            self._bound_symbols = res.union(
                *[sym.free_symbols for sym in self._vals.keys()])
        return self._bound_symbols