def set_edwards_eq(*, a=1, c=1, d=1):
    """Return equation that defines Edwards curve.

    To check if point is on curve.
    Edwards curve requires c, d params: x**2 + y**2 == c**2 * (1 + d * x**2 * y**2)
    Twisted curve requires a, d params: a * x**2 + y**2 == 1 + d * x**2 * y**2
    """

    def edwards_eq(x, y):
        return a * x ** 2 + y ** 2 == c ** 2 * (1 + d * x ** 2 * y ** 2)

    return edwards_eq