'Return a set of symbols other than the bound symbols used to
parametrically define the Curve.
Examples
>>> from sympy.abc import t, a
>>> from sympy.geometry import Curve
>>> Curve((t, t**2), (t, 0, 2)).free_symbols
set()
>>> Curve((t, t**2), (t, a, 2)).free_symbols
{a}'

@property
def free_symbols(self):

    free = set()
    for a in (self.functions + self.limits[1:]):
        free |= a.free_symbols
    free = free.difference({self.parameter})
    return free
