'Free symbols of the domain of ``self``.
Examples
>>> from sympy import Poly
>>> from sympy.abc import x, y
>>> Poly(x**2 + 1).free_symbols_in_domain
set()
>>> Poly(x**2 + y).free_symbols_in_domain
set()
>>> Poly(x**2 + y, x).free_symbols_in_domain
{y}'

@property
def free_symbols_in_domain(self):

    (domain, symbols) = (self.rep.dom, set())
    if domain.is_Composite:
        for gen in domain.symbols:
            symbols |= gen.free_symbols
    elif domain.is_EX:
        for coeff in self.coeffs():
            symbols |= coeff.free_symbols
    return symbols
