'This method returns the symbols in the object, excluding those
that take on a specific value (i.e. the dummy symbols).
Examples
>>> from sympy import SeqFormula
>>> from sympy.abc import n, m
>>> SeqFormula(m*n**2, (n, 0, 5)).free_symbols
{m}'

@property
def free_symbols(self):

    return set((j for i in self.args for j in i.free_symbols.difference(self.variables)))
