Set of free SymPy symbols contained within the equation.

def free_symbols(self):
        """Set of free SymPy symbols contained within the equation."""
        try:
            lhs_syms = self.lhs.free_symbols
        except AttributeError:
            lhs_syms = set()
        try:
            rhs_syms = self.rhs.free_symbols
        except AttributeError:
            rhs_syms = set()
        return lhs_syms | rhs_syms